Add whether the user has a password set in user APIs

https://redmine.teklia.com/issues/7318

To better handle the case where a user has registered without a password, we need a new has_usable_password attribute on the UserSerializer, which will add it to RetrieveUser, PartialUpdateUser and UpdateUser. This maps directly to the Django User.has_usable_password method, so this can be done directly with a BooleanField in DRF.

Existing tests will likely only include "has_usable_password": True. Please add a unit test to show that has_usable_password can be False as well.