Updated model. Started with tests.
minor
Almost made DRF work with nested url's. so close...
branches are now shown only for project. /project/1/branches/ lists all branches that have .project_ref = 1.
some tests.
Meta-model and symbol models removed from django model. They are now regular classes because they are not persisted in database. Made model serializers for those classes.
Removed their old serializers/viewsers and routers.
reworked project. meta-model and symbol models are now regular model classes (not django.model!).
implemented branch revision state building. added some tests.
added building of sandbox state implementation
fixed state building utils, so that they do not ignore previous state and can work for sandbox state building.
sandbox state building test and bug fixes.
Merge branch 'develop' into feature/dbmodel

Conflicts:
	fennec/settings.py
Merge branch 'feature/dbmodel' into develop