Implement bulk merge action

Created by: eseliger

https://user-images.githubusercontent.com/19534377/121703936-93b5af80-cad3-11eb-9666-64d3da4f9a99.mov

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