Enhance the /blocked_by, /blocks, and /unlink quick actions to support MR dependencies

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What does this MR do and why?

Enhance the /blocked_by, /blocks, and /unlink quick actions to support MR dependencies

The /blocked_by & /blocks quick actions allow for marking an issue or epic as being blocked by or blocking another. Update it to also work in the merge request context to set one MR as being blocked by or blocking another.

Add support for removing dependency links between merge requests using the /unlink quick action.

Signed-off-by: Patrick Talbert ptalbert@redhat.com