Add a new `/create_branch` quick action

EE MR: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/3629

Originally opened at https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10384 by @nurmuhammadsirat.

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What does this MR do?

Allows to create a branch using a quick action. The default branch name can be overriden.

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

I've changed how the JS handles quick actions because that was silly and since this action is quite different from the other ones (it doesn't act on the noteable directly).

Why was this MR needed?

Screenshots

Create a branch using the default name

Create a branch using the default name + other actions

Create a branch using the default name + other actions + comment

Create a branch using a custom name

Create a branch using a custom name that is invalid

Create a branch using a custom name that is invalid + other actions

Create a branch using a custom name that is invalid + comment

Step	Screenshot
Step 1
Step 2
Step 3
Step 4

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #27801 (moved)