use more reasonable default for `UserMoveInParams.begin_membership`
use more reasonable default for `UserMoveInParams.begin_membership`

The default in the `lib` function is `True` as well, so that makes
more sense.

I've checked that there is no entry in the database without this
parameter explicitly set.

agdsn/pycroft,agdsn/pycroft,agdsn/pycroft,agdsn/pycroft,agdsn/pycroft