Cleanups on models
Django 1.5: used the receiver decorator

Mostly syntactic sugar to allow connecting various signals to the same
handler. Other signal handlers have been left intact.

Fixes evernote#176.

Conflicts:
	pootle/apps/pootle_project/models.py

Cherry-Picked-From: 97eea7338c36678f2eadd2656f239efeb4eada24