Enhance checks for companion pull requests

[joao-paulo-parity]

At the moment the bot only cares about Substrate -> Polkadot with the implementation being hardcoded for those two. Polkadot is, however, not the only dependent and CI will be supporting more integrations in the future: paritytech/substrate#9010.

https://github.com/paritytech/ci_cd/issues/159#issue-939082694 talks about some issues which could be avoided by having more checks for any companions; the following checks would be triggered when someone comments bot merge:

• The companion PR should be mergeable (available in the Github REST API as mergeable) (Merge all companions once bot merge happens #313)
• The companion PR is not in an organization ("Allow edits from maintainers" should work on organisation repositories isaacs/github#1681 (comment) for context) (Merge all companions once bot merge happens #313)
• The companion PR does not change workflow files (More helpful message for corner case regarding updates to workflow files #302)

[joao-paulo-parity]

Since the remaining item (#302) already has a task for it, I'll be closing this issue.