Fall back to In-Reply-To and References headers when sub-addressing is not available

_Originally opened at !3024 by @dabit._

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Fixes #2364 

Summary of the changes:
- No more need to have the `%{key}` placeholder in the `incoming_email.address`
- The fallback message id format is `reply-[key]@[gitlab_host]` (reminder: it doesn't have to be a real email address)
- The fallback message id that includes the reply key is added to both `References` header
- Documentation for the "Reply by email" feature updated

See merge request !3305

Fix Raw / Rendered diff producing different results on merge requests

The raw diff endpoint (`merge_requests/####.diff`) produces a diff based on `target_branch` instead of the original branching point. Same is valid for `merge_requests/####.patch`.

Fixes #14680, #3017

See merge request !3450

Refactor builds badge, encapsulate inside a class

This merge requests attempts to introduce interface for all badges.
Currently we only have a build badge, but other badges are in plans,
like coverage badge.

See merge request !3403

Signed-off-by: Rémy Coutable <remy@rymai.me>

Functional closed sidebar

Adds functionality to include the closed sidebar.

When sidebar is closed the details will be visible once changed in open sidebar.

This MR fixes some bugs and adds some features from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3175

Fixes: #13594  

See merge request !3411

Fixed issue with notification settings not saving

Closes #14736

See merge request !3452

Closes #14736

Remove redundant `require`s from Banzai files

We're trying to avoid circular dependency errors.

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/13521

Sentry: https://sentry.gitlap.com/gitlab/gitlabcom/issues/8/

See merge request !3391

Dropdown bug fixes

- Added back upcoming to milestone filter

![Screen_Shot_2016-03-23_at_16.45.22](/uploads/8c31eff7e0984aad17881f6397ff17cd/Screen_Shot_2016-03-23_at_16.45.22.png)

- Fixed bug that would cause the currently selected value to disappear on filters
- Fixed bug that throw an error when filtering by upcoming when there is only a milestone in the past

Closes #14536

See merge request !3368

Gracefully handle notes on deleted commits in merge requests

Closes #3250

See merge request !3402

api - expose label description

Resolves #14415

See merge request !3314

Ensuring Merge Request API returns boolean values for work_in_progress

Fixes #14692.

See merge request !3432

Handle nil descriptions in Slack issue messages

Closes #14676

See merge request !3420

Updated gitlab-shell to 2.7

This is done to include the changes introduced in
https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/42.

See merge request !3437

This is done to include the changes introduced in
https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/42.

The frontend will then always use the name as the ID - like previous

Use raw SQL instead of Rails models in migrations where appropriate

Closes #14694.

See merge request !3429

Fixes tests

Added back upcoming to milestone filter
Fixed bug that would cause the currently selected value to disappear on filters
Fixed bug that throw an error when filtering by upcoming when there is only a milestone in the past

Fix order of steps to prevent PostgreSQL errors when running migration

Supersedes https://github.com/gitlabhq/gitlabhq/pull/10100.

/cc @stanhu @rodrigomanhaes

See merge request !3355

Fix error when visiting CI root path

Closes #14528, closes #14687

See merge request !3377