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
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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
ed19b9cc43