Github bot for producing PR reports

[g4s8]

It would be great to have GitHub app to use it instead of GitHub actions or other manual CI scripts. It can look similar to Codecov reports, e.g. see this comment: https://github.com/artipie/http/pull/204#issuecomment-661751762

The advantages:

• Aibolit automatically setup itself on auth: user clicks "install" button on Aibolit web page, it redirects to GitHub OAuth page, user select repository and clicks allow, then Aibolit has permissions to setup webhooks for repository and can run builds on some events (e.g. new PR event).
• Can generate cool reports: it would be more informative to have reports in PR comments. Now It's not really useful to open build logs of CI to check warnings text.
• Repo admin may decide how to deal with these reports: he/she can "require" this check to be passed in repo settings; he/she may log in to aibolit website and change behavior, e.g. minimum required score ration to make the build green. (see Codecov: it's red only when some threshold is reached, not for each minor issue).