Several cleanups on pootle_notifications views and templates

PEP8 cleanups.
Put imports in the right order.
Remove unnecessary or unused code.
title template var is used on RSS so set it always.
Rearrange code to make it more readable and efficient.
Remove title in template since users can see its location in breadcrumbs.
Remove language and project filters in get_recipients()

Quoted from Julen comment:

  Just note that some functionality of the pootle_notifications app is broken
  since we removed the 'Quick Links' feature in 77e4a79 as part of bug 2531
  — lang_filter and proj_filter won't have any effects.