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.
Link to the right web property
Allow options to be passed to select2.
Replace == operator and other minor fixes

* replaced == operator with equality operator;
* other minor fixes.
makeSelectableInput requires an options object.
Fix: add missing parentheses on multi-exception except clause
Multiple cleanups
Renamed legalpages to staticpages.

The old name has no sense for the upcoming functionalities.

Fixes bug 2826.
Minor fixes
Moved common static page functionality into an abstract page.

Fixes bug 2827.
Reordered fields, no functional change.
Provide default value for possible backwards migrations.
Removed unneeded check.

We can safely assume Django is > 1.3
Split except clause for handling different exceptions
Move check for accepted markup filter names to get_markup_filter_name()
Add get_markup_filter()

Now get_markup_filter_name() only returns a nice version of the filter name
for showing on the UI.
Fall back to HTML markup when markup configured module is missing

This fixes bug #2380.
spelling fix
update from wiki, remove obsolete ref, link to translate docs