[#642] Add dangerfile

Description

This adds some trivial checks by Danger. This tool looks promising because it can easily do things that we couldn't handle properly yet, like checking fixup commits presence.

I do not cause CI failure when Dangerbot finds crucial problems, because some problems are fine while the work is in progress, and keeping CI red during that and getting constant notifications would be annoying. Until we find a better solution, let's treat Dangerbot checks similarly to our MR checklist - the one who merges is responsible for ensuring that all marks are checked.

Related issue(s)

Resolves #642 (closed)

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• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
  • If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

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