[#730] Include dangerbot workarounds in messages

Description

Problem: we have workarounds for commit style checks, but those can only be discovered by inspecting the source code.

Solution: mention workarounds directly in dangerbot messages.

I have also moved some larger checks to separate files/functions to make working on them a bit more convenient.

Partially based on !1073 (closed).

To see this in action, see !1208 (closed)

Related issue(s)

Resolves #730 (closed)

✅ Checklist for your Merge Request

Related changes (conditional)

• 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.

• Documentation

  • I checked whether I should update the docs and did so if necessary:
    • Root README and other README.md files
    • Haddock
    • docs/
  • I updated changelog files of all affected packages released to Hackage if my changes are externally visible.

N/A

Stylistic guide (mandatory)

• My commits comply with the following policy.
• My code complies with the style guide.