enforce min password length on both FacilityUser.set_password and FacilityUserForm
disable checking of raw_password if it's None
correct failing tests from password length enforcement

Conflicts:
	kalite/main/tests/api_tests.py
raise validation error instead of an assert when enforcing pw length
enforce pw length in changelocalpassword
in settings, turn MIN_PASSWORD_LENGTH to more generic PASSWORD_CONSTRAINTS
factor out password checking logic into verify_raw_password function
use verify_raw_password for changelocalpassword