Enable team leaders to act on a broken production release

[joao-paulo-parity]

parity-processbot sees a lot of activity on Substrate and Polkadot, to the point of having it be broken for some critical scenario would possibly hinder the developers' productivity.

This project's maintainers currently lack guarantees about their changes working properly due to the code not being reasonably tested (#251), therefore it's not unlikely for the doom scenario to accidentally come up. Even if the code was extensively tested, though, it would still be good to give team leads the power to act on brokenness given how frequently the bot is used for the core projects. It should also taken into account that the maintainers might not always be available to fix those problems promptly for some reason (e.g. sick leave).

Ideally the solution is something easy to use e.g. a Github Action Worflow which can be triggered with a button or creating a issue in this repository with "bot rollback [version]".

Note that while this would affect the bot "globally", as far as I know it's only used on Polkadot and Substrate, therefore I can assume everyone would be fine with rolling back to a previous working release.

related to https://github.com/paritytech/devops/issues/869

[joao-paulo-parity]

This is already achievable through CI.