Automatically retarget merge requests [RUN ALL RSPEC] [RUN AS-IF-FOSS]

What does this MR do?

It is common to have a number of merge requests in a chain, when one depends on another. For example:

• Merge Request A: merge feature-A into master
• Merge Request B: merge feature-B into feature-A

We can distinguish two workflows:

• the MR A is merged into master first, and then MR B is retargeted onto master
• the MR B is merged into feature-A branch, and then MR A is merged into master

Upon merge of MR A does automatically retarget the MR B onto master. This relieves user from having to perform this operation manually.

This feature works only in following cases:

• The MR A needs to be in a main project (forks not supported), as we GitLab is cannot change target project of MR B
• The MR A deletes branch on merge
• Only 4 MR's of type B are retargeted automatically this way
• A note of changing target branch is created automatically by system.

Related to: #320902 (closed).

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content