Remove icons from some buttons which already has text

Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/18324


* remove plus icon from new milestone button on group milestone page
* remove plus icon from new project button on group projects page
* remove plus icon from milestone page

See merge request !5050

Expire the branch cache after `git gc` runs

Due to a stale NFS cache, it's possible that a branch lookup fails while `git gc` is running and causes missing branches in merge requests.

I'm not totally convinced this is the right solution, but since we and our customers are experiencing this issue quite frequently, I'm taking a stab at it.

Possible workaround for #15392


See merge request !5160

Be explicit on merge request discussion variables

## What does this MR do?

To avoid conditionals and to messing with request.format and accept headers to know in which format we're going to response I've decided to be explicit in when we need the discussion variables

## Why was this MR needed?

Solve a bug https://sentry.gitlap.com/gitlab/staginggitlabcom/issues/8492/

## What are the relevant issue numbers?

Closes #19702 

## 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
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5204

Style of import project buttons were fixed in the new project page.

## What does this MR do?
I have fixed style of import project buttons in the new project page.

## Are there points in the code the reviewer needs to double check?
I don't think so.

## Why was this MR needed?
Because the buttons area hasn't looked well. Before this, it was scattered.

## What are the relevant issue numbers?
Fixes #19467 

## Screenshots (if relevant)
![Screen_Shot_2016-07-11_at_17.12.11](/uploads/35918f16b1b4d671ad33faf6caf530a2/Screen_Shot_2016-07-11_at_17.12.11.png)
![Screen_Shot_2016-07-11_at_17.11.24](/uploads/3030e1ecb7d109fbf4552987b4bfb242/Screen_Shot_2016-07-11_at_17.11.24.png)
![Screen_Shot_2016-07-11_at_17.11.50](/uploads/eb8ede86c256a2356b0210f3876c0143/Screen_Shot_2016-07-11_at_17.11.50.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
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !5183

Due to a stale NFS cache, it's possible that a branch lookup fails
while `git gc` is running and causes missing branches in merge requests.

Possible workaround for #15392

Fix expandable diffs

## What does this MR do?

Fix expandable diffs and the master build.

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

Nope.

## Why was this MR needed?

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4776#note_13003494

Doesn't need a CHANGELOG entry etc.

See merge request !5200

Add rule about adding new header tab to the ui guide

To address cases like https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3749#note_12994215 

See merge request !5188