Unify response from UI and API for rebase operation

What does this MR do and why?

Contributes #386537 (closed)

Problem

1. The rebase validation logic is duplicated in two places
2. Rebase action in controller returns a html 404 page, instead of a json response with an error.

Solution

• Move shared logic into MergeRequests::RebaseService.
• Change the rebase response in controller to return json error.

Screenshots or screen recordings

Rebase button on the merge request page

Before fix	After fix

How to set up and validate locally

1. Go to Project Settings -> Merge requests and set Fast-forward merge option.
2. Create a merge request that is behind of the default branch
3. Make merge request source branch protected with force push restriction
4. Press Rebase button on the merge request page
5. See an error

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.