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
Random cleanups
Docs: Several changes in release notes
Added commands and store actions logging

Cherry-Picked-From:
  evernote@d8d70bfc195747c020f46ac82f3a38ec1f894349
  evernote@0388d6862119f45932936c580281b1f88c63f942
  evernote@45cf6acb49951db96d51c96340290485cb717e30
  evernote@c114e0a44c278723d500aac2ebddcd61593702e8
  evernote@79d68bdfc3fa2d6e22e31c4b95b9530a8e3198b3
  evernote@bbb2ffaba090e85aae7b634de6fc19c574bccc9a
  evernote@00544187ff652726cea4f4310e9fe9dfdb93d7fa
  evernote@6b263d0426836464d91fa4dbb0d602d93eef5b54
  evernote@62901f6bed0ee24eaec23c94cdf63bd218d4525d

Pick notes:
This is a squash of the logging changes by @ta2-1, including all
the followup fixes by @julen.

We also import pootle_misc.log instead of pootle_misc.log.<action>
to make things more readable.

Also completes partial pick of evernote@17ab08b that was started in
c5dc53de0472ad5cf63a5d729ebabd692d8c1996
Cleanup: moved `log` to `pootle.core`

Cherry-Picked-From: 14102e55d86a62a57f2da05a90e2e864b574c151
Fix log formatting

Conflicts:
	pootle/apps/pootle_misc/log.py
Added logging defaults to the sample config file
Ignore 'log' directory

Cherry-Picked-From: 9058b17b3d815ea7d8958c385cf447365b71f82b
Added default logging configuration for development
Moved log directory settings definition and documented its purpose

Conflicts:
	docs/server/settings.rst
	pootle/settings/20-backends.conf
Use os.path.join instead
store: log UNIT_OBSOLETE action

Partial-Pick: d49cc1b5e1c8359e6ce43bb251ad633e535ff6ee