WIP: resolves "Allow splitting MRs"

What does this MR do?

This MR allows the user to split a current MR into a smaller MR by commit. There is now a button in the app/views/projects/commits/_commit.html.haml which will link to a new MR which has all the information pre-filled, this information is heavily based on the current MR that you are looking at.

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

Yes, the following:

• app/views/projects/commits/_commit.html.haml - this is used in a number of places, the question is... is this the right place to put the button and if so, what should the restrictions be in order to see button?

• The UX is not fantastic, at the moment there is just a button to allow you to split the MR and the information on that page assumes knowledge of what is going on. Maybe this is something that @dimitrieh could comment on eventually. (I know there is some other ideas in the pipeline)

• At the moment the only setting I have copied is remove_source_branch is there others which I may have missed @smcgivern + @jarka?

Why was this MR needed?

It allows you to split up large pieces of work into more manageable and deployable chunks. I think this is a fantastic addition to the platform.

Screenshots (if relevant)

Existing MR:

New Split MR:

Complete Split MR:

Screencast:

https://drive.google.com/file/d/1LjncszRVQF_Cv2WU_0JCRryLYdYR3KGV/view

(it's painfully slow on my local machine so stick with it 😉)

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
• Internationalization required/considered

What are the relevant issue numbers?

Closes #42078 (moved)