[#192] Initial commit for packaging models.py

Will have to be checked on:
- Circular imports
- User relations (import from django.contrib.auth instead of '')
- Functions outside of classes to models_utils.py
  - Clean up models_utils.py
[#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
[#192] Cleanup of a lot of model's "static" variables

By "static" I mean data that's used in a static manner, like choices
tuples. Many of them are made into Model attributes.
[#192] Merge branch 'develop' into feature/192_refactor_models

Conflicts:
	akvo/rsr/models.py

Delete models.py and incorporate the needed changes from models.py into
the relevant files under models/
[#192] Modify imports

Make imports from rsr relative
[#192] Code cleanup

Cutting overly long lines and some other small cleanup actions
[#192] Newlines at end of files

And a couple of other small lint fixes
Merge pull request #719 from akvo/feature/192_refactor_models

Feature/192 refactor models