buildchecker: pipeline failure detection and branch locking

Created by: bobheadxi

This will run as an automated cron every ~15mins, and it will do the following:

• get a page of recent builds and check if:
  • the most recent build is failed
  • that failure is consecutive
• if no issues detected, remove branch push restrictions
  • if restrictions were removed, post in slack
• if an issue is detected:
  • try and find github handles for the relevant authors of the commits
  • lock main branch: this is done by restricting push to these build authors + the devx team
  • post in slack if this is a new restriction (existing restrictions won't be overwritten)

I've tried to include some extensive testing because this is definitely not something we want misbehaving 😬

Close https://github.com/sourcegraph/sourcegraph/issues/25482 and https://github.com/sourcegraph/sourcegraph/issues/28834 and https://github.com/sourcegraph/sourcegraph/issues/29005

Notes:

• A lot of the diff is just generated API interaction recordings
• A major caveat is that the API does not currently allow us to set "require pull request" (!!!!) https://github.com/sourcegraph/sourcegraph/pull/28759/files#r767060192 . Is this a blocker? This is fixed by using the GraphQL API for this part workaround no longer needed due to https://github.com/sourcegraph/sourcegraph/issues/29005
• Currently does not automate incident creation, but does encourage it in the slack message
• Currently does not explicitly tag users, since that is complicated and might be better served by https://github.com/sourcegraph/sourcegraph/issues/25454. A slack message is posted though