Replace static fixture by generated one in issue_spec.js

## What does this MR do?

- clean up `issue_spec.js`
- introduce an alternative approach to #19445
  - rename `rake teaspoon` to `rake teaspoon:tests`
  - introduce `rake teaspoon:fixtures` which generates fixtures using RSpec
  - introduce `rake teaspoon` which runs `rake teaspoon:fixtures` and `rake teaspoon:tests`

## Why was this MR needed?

- many duplications
- missing existence checks
- missing conditions
- static fixtures don't match real views

## Reasoning

I want to explain some of my decisions here, so that they stay visible for future discussions.

### Why not HAML?

- same number of HAML templates as number of fixtures (many input files)
- embedded logic less readable
- can not be rendered by JavaScript (because of inline Ruby)

### Why RSpec?

- real controllers for fixtures
- spys available for mocking
- easily report failed fixture generations

### Why not magic_lamp? (#19445)

- introduces another dependency/tool
- needs to run a server concurrently to teaspoon
- makes it harder to use a JavaScript test runner
- static HTML files serve faster

See merge request !6059

Modify GitHub importer to be retryable

Closes #23533

See merge request !7003

Only show one error message for an invalid email

Changes it to only validate the notification_email format if it's different from email.

Closes #5905

See merge request !7158

Stop unauthorized users dragging on milestone page

Closes #13670

See merge request !7113

Document admin access requirement for /deploy_keys endpoint

Closes #23807

See merge request !7164

Fix and improve `Sortable.highest_label_priority`

Closes #23928

See merge request !7165

API: Fix booleans not recognized as such when using the `to_boolean` helper

Fixes #22831
Fixes #23890

See merge request !7149

Jira refactoring to jira gem

part of gitlab-org/gitlab-ee#1008

Original MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/2572/commits

See merge request !6598

Prevent a spec from failing in `spec/features/merge_requests/diff_notes_resolve_spec.rb`

## What does this MR do?

Fixes a failing spec in `spec/features/merge_requests/diff_notes_resolve_spec.rb`

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

Tests failing on master. RSpec executes inner `before` blocks after outer `before` blocks, so the second note was being added *after* loading the page, and then being pulled in by running JS on the page, causing intermittent failures if we clicked the resolve button before the note was loaded.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

Closes #23875

See merge request !7150

Merge branch 'bugfix/dragging_milestones' of gitlab.com:blackst0ne/gitlab-ce into bugfix/dragging_milestones

Fix couple of GitHub importing bugs

Fix a bug in GH comment importing and label applying for imported MRs.

See merge request !7139

edited stylesheet with mr modal fixes and added to changelog

## What does this MR do?
It shortens the modal command merge info elements.

## Are there points in the code the reviewer needs to double check?
I used the `.dark` class.. is that correct usage?
Made no issue as this is a minor upgrade to UI

## Why was this MR needed?
Clipboard buttons could obscure information in the merge info modal, this way its clean.

## Screenshots (if relevant)

From:
![image](/uploads/99269c9a314c593cb06239c27565b07c/image.png)

To:
![image](/uploads/7e278a5aaa8c57270e8a320cc270b5d8/image.png)


## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?
None, its a quick fix

See merge request !7145

Add section about contributing to README

Closes #23830

See merge request !7137

Get fix for teaspoon tests

Replaced jquery.cookie with js.cookie

## What does this MR do?

Fixes currently failing `teaspoon` build by replacing the last reference to `jquery.cookie` with `js.cookie`.

## Are there points in the code the reviewer needs to double check?

## Why was this MR needed?

`master` failing.

## Screenshots (if relevant)

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG.md) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?

See merge request !7160

Getting latest CHANGELOG so I can resolve merge conflict

If notification_email is blank, it's set from email. If an admin
attempted to create a user with an invalid email, an error would be
displayed for both fields. Only validate the notification_email if it's
different from email.

Enable linting for ES6 files

## What does this MR do?

Add `.js.es6` to file extensions checked by ESLint.

## What are the relevant issue numbers?

Follows !5445

See merge request !7156

Document CI pipelines settings

See merge request !5847