Fix "Resolve conflicts" button not appearing for some users
Previously the frontend assumed that the user had to be able to merge to
that project in order to resolve conflicts. However, this is overly
restrictive, as the user only has to be able to push to the source
branch. In fact, appending the text /conflicts to the merge request
would bring up the conflict resolution page.
This confusion happens when a project contains a protected branch that only allows maintainers to push. Users with Developer access no longer have permission to merge, but they still can create branches in that project.
To fix this issue, we now loosen the permission check for showing the "Resolve conflicts"
button and only check for access to push to the source branch. This is consistent with what the
backend does in MergeRequests::Conflicts::ListService#can_be_resolved_by?.