[#192] Fix models rsr.models package

Add app_label = 'rsr' to all Meta class definitions

Change imports of models to be relative, e.g. from .goal import instead
of from akvo.rsr.goal import

Add missing imports into __init__.py

Add __all__ to __init__.py

Move some "constants" from model_utils.py in their respective classes
[#192] Fix rsr.filters and rsr.forms to work with rsr.models package

Also delete models.py
[#192] Change import of User model in rest user view and serializer