Denormalize: Add a translation_project field to Directory

[unho]

This will allow to get rid of some ugly code. Directories probably won't change TP and this will allow to simplify some queries like

pootle/pootle/apps/pootle_project/views.py

Lines 135 to 137 in 1e0a852

	"%stranslationproject" % tp_prefix,
	"%stranslationproject" % tp_prefix,
	"%stranslationproject__language" % tp_prefix]

or

pootle/pootle/apps/pootle_app/models/directory.py

Lines 100 to 113 in c53eca1

	@cached_property
	def translation_project(self):
	"""Returns the translation project belonging to this directory."""
	if self.is_language() or self.is_project():
	return None
	if self.is_translationproject():
	return self.translationproject
	aux_dir = self
	while not aux_dir.is_translationproject() and aux_dir.parent is not None:
	aux_dir = aux_dir.parent
	return aux_dir.translationproject