Make MyPy happy

See merge request !22

db = SQLAlchemy() should be enough to already infer the type of the variable

Update to solve the LSP errors via typing changes and not by code changes.
This undos LSP-violation related code changes.

The argument can only be a string or None, because
dateime.from_isoformat expects only a string.

According to mypy the previous annotation violated the LSP.
We now use the types of the supertype method and return a
datetime object only if the argument was a datetime-object.

According to mypy the previous annotation violated the LSP.
Therefore, we use the types of the supertype method and check for
correct types in the method.

Introduce Pipenv

See merge request !21

Add mypy configuration in pyproject.toml

Replace requirements.txt with pipenv for consistent development
environments.
Added additional development dependencies.

It is now available at /info-4-devs/

Add documentation

See merge request !20

Resolve "Add an icon for the account type "Uni-ID""

Closes #40

See merge request !19

Fix link

See merge request !18

Introduce apple-touch-icon to header.

See merge request !17

Add link to the daily Mensa menu to one of the pages

See merge request !16