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

Why was this MR needed?

It improves UX of branch management.

Screenshots (if relevant)

To be added.

Does this MR meet the acceptance criteria?

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

What are the relevant issue numbers?

Closes #21143 (closed)

mentioned in issue #21143 (closed)

added 4 commits

• 288f0db5 - Replace 'project/star.feature' spinach test with an rspec analog
• 573f7056 - Merge remote-tracking branch 'upstream/master'
• 1f500e44 - Merge remote-tracking branch 'upstream/master'
• bd52cc4b - Add changelog entry

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added 1 commit

• fd56c7f1 - Add a changelog entry

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changed the description

marked the checklist item Changelog entry added, if necessary as completed

added 1 commit

• 3c0ee95e - Add input fields to the dropdown

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marked the checklist item Add field for branch name as completed

added 1 commit

• 8eb675ff - Fix autoclosing dropdown

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marked the checklist item Add field for source branch as completed

added 1 commit

• d7e80207 - Init JS functions

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marked the checklist item Ability to create a branch with a custom name as completed

marked the checklist item Ability to create a MR with with a custom branch name as completed

marked the checklist item Change URL depending on branch name as completed

changed the description

added 1 commit

• b8ccc37f - Add ability to create MR with custom branch name

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changed the description

changed the description

@smcgivern

Our current logic of generating an MR description is based on a branch name.

If a user changes a branch name from a default to a custom one, the current implementation of the assign_title_and_description method will not append the Closes iid text to the MR description.

What should I do with it?
I personally prefer always pass an iid of an issue and always append the Closes xxx string, but there might be some reasons why we don't do that right now.