Fix the UserSerializer class

During black box testing of FR5 (View Workout) a bug in the class-level validation on the UserSerializer class was discovered. When accepting a coach offer, user validation would fail, complaining about the password being None.

The changes made to the initial implementation is know mostly reverted. Class-level validation is removed in favour of field-level validation for the password field. The implementation of the password validation, however, remains the same.

Additionally, default values for related required fields are know declared using the extra_kwargs attribute.

Unit tests for the UserSerializer class is also updated, to reflect the changes.