Merge branch '20189-markdown-video-doesn-t-work-when-the-referenced-video-file-is-in-same-repo' into 'master'

Ensure relative paths for video are rewritten as we do for images

## What does this MR do?

This ensures that path to videos are rewritten as we do for images.

## Are there points in the code the reviewer needs to double check?

I've decided to add a new `Blob#video?` method for simplicity's sake but this should probably be added to `Gitlab::Git::Blob`.

## What are the relevant issue numbers?

Fixes #20189.

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- Tests
  - [x] Added for this feature/bug
  - [ ] 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 !5474