`#referenced_merge_requests` preloaded too many associations. Award emoji, for
instance, are completely unnecessary here.
`#closed_by_merge_requests` had the opposite problem: `#all_references` needs
each item's author, and these weren't preloaded.
These methods don't really need to be on the Issue model. Issue#related_branches
can also be moved to a service, but we can do that in a separate commit.
This commit does not change any behaviour; it just moves code around, renames
the service, and refactors the specs.
