merged Field and FieldGroup classes
Merge branch 'feature/fields_refactor' of github.com:globaleaks/GLBackend into feature/fields_refactor

Conflicts:
	docs/ng-fields-spec.md
	globaleaks/handlers/admin/field.py
	globaleaks/models/__init__.py
	globaleaks/rest/requests.py
	globaleaks/tests/handlers/admin/test_admin.py
	globaleaks/tests/helpers.py
	globaleaks/tests/test_models.py