Add ability to disable projects (#97)

Conflicts:
	pootle/apps/pootle_project/models.py
	pootle/core/decorators.py
TranslationProject: added convenience `disabled()` manager method
translationproject: Add the ability to disable a translation project

Cherry-Picked-From: a6b5dd9db6e6495444730d4e33b354315e86e967
Disabled translation projects aren't to be updated

No log error messages while update_stores after whole directory
of translation project was removed

Cherry-Picked-From: 1ad4f599abe8c870c3793bc7facd95fed031aeb4
Tests: added Arabic language fixture
Tests: allow passing options when creating projects
Tests: added disabled tutorial project
Tests: added disabled TP
Tests: check `get_path_obj()` behavior
Tests: allow having stores with the same name