Add textarea autoresize after comment

## What does this MR do?
Adds an autoresize after a comment is posted

## Are there points in the code the reviewer needs to double check?
Shouldn't be

## Why was this MR needed?
To keep the UI consistent (the same as if you were to refresh it)

## Screenshots (if relevant)
Before:
![IyCczzXfaf](/uploads/2229bad47e3f3821f1efaa6868a52025/IyCczzXfaf.gif)

After:
![E6O0kRBrJq](/uploads/4aec6b436ff172ba281c2c2a4481906d/E6O0kRBrJq.gif)

## 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
  - [x] All builds are passing
- [x] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [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)

## What are the relevant issue numbers?
Closes #21667

See merge request !6152