Cleanup: use Django's own `cached_property`

No need to duplicate functionality here. Besides, Django's implementation
is faster
http://ericplumb.com/blog/understanding-djangos-cached_property-decorator.html

Conflicts:
	pootle/apps/pootle_misc/util.py
	pootle/apps/pootle_project/models.py
	pootle/apps/pootle_store/models.py
	pootle/apps/pootle_translationproject/models.py
	pootle/models/user.py
Cleanup: removed unused imports