Resolve "Customize branch name when using create branch in an issue"

What does this MR do?

This MR adds an ability to type a custom branch name in the Create merge request dropdown placed in an issue.

TODO

  • Add backend specs
  • Add frontend specs

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

create_mr_button

Why was this MR needed?

It improves UX of branch management.

Screenshots (if relevant)

To be added.

Does this MR meet the acceptance criteria?

What are the relevant issue numbers?

Closes #21143 (moved)

Edited by username-removed-86853