Add `first_or_none`

[dekoza]

Is your feature request related to a problem? Please describe.
I'm coming from Django and I'm used to having first() return None if there are no matches. Ormar should either replicate this behavior or introduce first_or_none method to go in sync with other _or_none methods.

Side note: Documentation for get() and get_or_none() is misleading as it reads "Returns first row" which sadly is not true because it raises MultipleMatches if there are more rows matching criteria.

Describe the solution you'd like
I already have a PR adding first_or_none but the docs suggest opening an issue beforehand 😉

Side note: mypy's pre-commit hook yields a lot of errors from other code in the repo.