Completing the move to flake8-docstrings

[jerbob]

We've decided to add flake8-docstrings to project requirements for this repository.
We've agreed on the following list of errors to ignore:

ignore=
    P102,B311,W503,E226,S311,
    # Missing Docstrings
    D100,D104,D107,
    # Docstring Whitespace
    D203,D212,D214,D215,
    # Docstring Quotes
    D301,D302,
    # Docstring Content
    D400,D401,D402,D405,D406,D407,D408,D409,D410,D411,D412,D413, D414

[lemonsaurus]

We've decided not to ignore D202 and D204. We've also decided to ignore D400. I've edited the OP to reflect this. See https://github.com/python-discord/organisation/issues/53#issuecomment-486371508 for more info.

[sco1]

Closing in favor of the more overarching issue here: #384