Move Markdown/reference logic from `Gitlab::Markdown` to `Banzai`

- Moves from `Gitlab::Markdown` to `Banzai`
- Moves filters and pipelines into their own namespace: `Banzai::Filter` and `Banzai::Pipeline`
- No more `autoload`!
- Split up `Gitlab::ReferenceExtractor` into `Banzai::ReferenceExtractor` and `Gitlab::ReferenceExtractor`
- Replace `something(load_lazy_references: true)` by `Gitlab::ReferenceExtractor.lazily { something }`

Goes from:

```ruby
def referenced_merge_requests
  references = [self, *notes].flat_map do |note|
    note.all_references(load_lazy_references: false).merge_requests
  end.uniq!

  Gitlab::Markdown::ReferenceFilter::LazyReference.load(references).uniq.sort_by(&:iid)
end
```

to

```ruby
def referenced_merge_requests
  Gitlab::ReferenceExtractor.lazily do
    [self, *notes].flat_map do |note|
      note.all_references.merge_requests
    end
  end.sort_by(&:iid)
end
```

See merge request !2027

Add user repository integrity check rake task

Corrupt repositories and stuck lock files can cause weird issues in
GitLab. Often we know which user is having these problems and then we
have to go hunt down which repository is causing it. Several times
recently that involved me running queries in the rails console to get
an array of projects and then writing a quick Ruby script to loop
through and run `git fsck`. This last time I also had to check for the
existence of `config.lock` and ref lock files. 

This rake task will eliminate all of those steps and allow an admin to
simply specify a username.

I also added the lock file checks to the existing `gitlab:repo:check`
task which goes through all projects. 

See merge request !2080

Increase fixed layout width to 1280px
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

cc @creamzy 

See merge request !2075

Fix inline panel heading height

The `line-height` was added so that headings in the panels on the settings page had the same height as headings elsewhere, but the style applied everywhere :)

Before:

(Look at the "Unstarted Issues" heading)

![Screen_Shot_2015-12-15_at_17.39.46](/uploads/9dbea4c8b28e3c43bb5d4b5226d7a1ec/Screen_Shot_2015-12-15_at_17.39.46.png)

![Screen_Shot_2015-12-15_at_17.40.19](/uploads/97898418e9a66954c02e8a68b21c08b9/Screen_Shot_2015-12-15_at_17.40.19.png)

After:

![Screen_Shot_2015-12-15_at_17.40.45](/uploads/5748aba032889053dd778b39e96fa301/Screen_Shot_2015-12-15_at_17.40.45.png)

![Screen_Shot_2015-12-15_at_17.40.40](/uploads/20240cdc7a6d5665cd32556d1a0e6c1d/Screen_Shot_2015-12-15_at_17.40.40.png)

cc @skyruler 

cc @rspeicher 8.3 please!

See merge request !2106

Add feature specs for note polling

Ref. #4032, !2084

cc @rspeicher 

See merge request !2086

This also increases capybara timeout to 15 seconds (note polling
interval). Capybara will look for new note for this period of time.

Ref. #4032, !2084

Fix note polling

Closes #4032

See merge request !2084

Preserve trailing new lines at the end of file on the online editor

Fixes #3784

Because Haml automatically indents the HTML source code, the contents
of whitespace-sensitive tags like pre and textarea can get screwed up.

See merge request !2099

Clean up CONTRIBUTING.md

* Use 80 chars width where possible
* Remove easyfix label reference
* Add new issue submission template
* Add long links to the bottom of the page

See merge request !2100

* Use 80 chars width where possible
* Remove easyfix label reference
* Add new issue submission template
* Add long links to the bottom of the page

Because Haml automatically indents the HTML source code, the contents
of whitespace-sensitive tags like pre and textarea can get screwed up.

update guides for feature proposals on the issue tracker

see also #4038 

cc @sytses 

See merge request !2094

Fix Typo [mailinglist to mailing list]

Closes: #4078 

See merge request !2098

[ci skip]

Add upgrade guide for 8.2 to 8.3

Making sure we don't forget mentioning the need to upgrade Redis from
!1888. Added this in preparation for 8.3.

[ci skip]

See merge request !2085

[ci skip]

[ci skip]

[ci skip]

[ci skip]

Suppress warning about missing `.gitlab-ci.yml` if builds are disabled

When user disables GitLab Ci Service in project's settings then warning
about missing `.gitlab-ci.yml` file should be supressed. This a matter
of user experience as stated in #3761 (closes #3761).

cc @ayufan 

See merge request !2014

[ci skip]