Add option to specify mandatory checks per project

[crtschin]

Add option to specify mandatory checks that need to be satisfied before merging.

• In the config.json it is now possible to add mandatory checks per project via a "checks" -> "mandatory" path.
• Each commit status update specifies a mandatory context key that is now parsed, see here.
• If context satisfies the check, by the check being the prefix of the given context, its status is added to the integration of that commit.
• Once all mandatory checks have succeeded, the PR is merged.

As an aside, I also updated the thread initialization to use Map instead of []. Though I'm open to dropping this change considering it doesn't do much if all repositories have the same owner.

[crtschin]

@OpsBotPrime merge

[OpsBotPrime]

Pull request approved for merge by @crtschin, rebasing now.

[OpsBotPrime]

Rebased as ef8f5dcde81869b392de8743e87fb0fe9d76ad6f, waiting for CI …

[OpsBotPrime]

CI job :yellow_circle: started.