Templates: implemented generic about page

Brings back stock Pootle content.

Fixes evernote/pootle#262.
WOKRING: Trying to fix this migration

I am not sure why it fails
Cleanup: Removed unused imports
Cleanup: Remove unused export code
Cleanup: Use reverse instead of hardcoded URLs
Some previous cleanups

Basically to reduce code length, improve readability and ease further
modifications.
Offline translation
Offline signals
Offline upload handling