Add Mypy

[JoeyStk]

Short description

This PR adds Mypy as a new dependency to our repository.

Proposed changes

• Add Mypy
• Add Mypy to CI
• Add Mypy shell script
• Fix (or mostly ignore) failing lines

Side effects

• The idea was to exclude files in the /cms/ or /api/v1/ repository. This doesn't fully work how I wanted to. Even after fiddling around for a few hours, I didn't get to exclude some files in /cms/models or in the plain /cms/ directory. If you want to, you can try it yourself and see if you can come up with something I didn't :)
• Additionally, the idea is to mostly ignore currently existing code but check mypy for new code, therefore the many ignores. Hopefully we can work our way forward from there

Resolved issues

Fixes: #657

[LeandraH]

Looks great! Should we add some information to our documentation?

[JoeyStk]

Should we add some information to our documentation?

Good point! I added it in this commit :)