Make kwargs for messages in assertions consistent

Right now the kwargs for custom assert messages are inconsistent, with 11 uses of `msg` and 13 uses of `message`. This makes it only use one name so it's easier to remember and you don't have to look up whether you're supposed to do `assert_foo(..., msg="bar")` or `assert_foo(..., message="bar")`.

I didn't touch the ones in deprecated_assertions.py because they're deprecated already.