Be more respectful of when workshifter sign out of a shift
Be more respectful of when workshifter sign out of a shift

Don't re-assign members to a shift once they have signed out
Slight change to _check_field_changed's function call, added a docstring
Create workshift instances in pre_save when going from inactive to active

knagra/farnsworth,knagra/farnsworth,knagra/farnsworth,knagra/farnsworth