Cleanups from landscape.io report

Missing imports, unnecessary imports, indents, comments...
Bugfix: Make Store use StoreManager

Fixes #3358.
Cleanup: Remove unused export code
Cleanup: Use reverse instead of hardcoded URLs
Cleanup: Move action decorators to pootle.core

They are going to be used by several apps in a close future.
Offline: Move offline translation and exporting to a new app

This will allow to enable or disable this feature in the feature.
Offline: Move signals and signals handling to the new app
Offline: Move upload handling to the new app
Offline: Disable if not present in INSTALLED_APPS
WOKRING: Trying to fix this migration

I am not sure why it fails
UNDO
Bugfix: Add the field in the right model

This is fallout from the bad pick of evernote/pootle@b47cd81a in c7cb7502.
UNDO not cool