Add support for inline videos in issue, MR and notes (on issue, commit, MR, and MR diff)

## What does this MR do?

It adds support for inline videos in issue, MR and notes (on issue, commit, MR, and MR diff). Most of the work was done by @hayesr in !3508 but a few improvements were still missing.

## Why was this MR needed?

To be able to play uploaded videos in GitLab!

## What are the relevant issue numbers?

Closes #4142.

## Screenshots

### Video players

![Screen_Shot_2016-07-19_at_18.44.09](/uploads/e85e531b455a41c3e66b26b356abaafd/Screen_Shot_2016-07-19_at_18.44.09.png)

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![Screen_Shot_2016-07-19_at_18.44.29](/uploads/05f52a812760210d1eae86a7f8fc48bc/Screen_Shot_2016-07-19_at_18.44.29.png)

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## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [x] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- Tests
  - [x] Test `VideoLinkFilter`
  - [x] Test in `spec/features/markdown_spec.rb`
  - [x] Improve `spec/uploaders/file_uploader_spec.rb`
  - [x] 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 !5215