Closes #29536

Clean up the events when the modal window is closed

[ci skip]

Now you can create a new file and select a target_branch != source_branch.
If the file that you want to create already exists on the target branch an error message is shown
A glDropdown is used to select and create a new branch instead of a text field.

Delegate a discussion in a merge request into a new issue.
The discussion wil be marked as resolved and a system note will be
added linking to the newly created issue.

center spinner
show placeholder text on new MR page
remove redundant `display: block` from li - they default to `list-style` already

Maintain emoji line-height in it's context to avoid cut-off,
(Windows 10) http://i.imgur.com/xmdOLgv.png

This also allows us to get rid of the small bodge for the cut-off in
Safari

 - gl_emoji for falling back to image/css-sprite when the browser
   doesn't support an emoji
 - Markdown rendering (Banzai filter)
 - Autocomplete
 - Award emoji menu
    - Perceived perf
    - Immediate response because we now build client-side
 - Update `digests.json` generation in gemojione rake task to be more
   useful and  include `unicodeVersion`

MR: !9437

See issues

 - #26371
 - #27250
 - #22474