Implement bulk merge action
Created by: eseliger
This PR implements a bulk merge action for changesets. It can be used to merge multiple open changesets with one operation. This PR contains:
- A new bulk operation type
- UI for running the merge bulk operation
- Code host integrations to trigger a merge
As discussed yesterday, this PR will:
- Detect failures that are likely non mergeable state errors and bail out if it finds them (ie no retries)
- Implement two merge strategies, merge and squash. This is not supported by bitbucket server and we fall back to merge commits always in that case.
Closes https://github.com/sourcegraph/sourcegraph/issues/20865