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 (closed) 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 (closed).

Screenshots

Video players

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Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • Test VideoLinkFilter
  • Test in spec/features/markdown_spec.rb
  • Improve spec/uploaders/file_uploader_spec.rb
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together