Add `css_overrides` block that will go below the `css` one

This way it will always override any Pootle default styles on any pages.
Make HR lines semitransparent so that they work against any colored background
Improve thumbnail styling in normal/hover/focused states
Make browser actions hidden by default to avoid flicker on data load
Prevent crazy collapsing/expanding of stats drawer when stats data is refreshed

Also, expand or collapse only the first time; if user prefers to have the stats collapsed,
leave it intact on stats reload.
Cleanup: s/mimetype/content_type/

The `mimetype` argument to `HttpResponse` is deprecated and will be
removed in Django 1.7.
Requirements: drop memcached in favor of Redis

Refs. #3351.
Address code arrangement/styling issues raised by Julen
Settings: Add logging settings to 20-backends.conf
Tests: fixed docstring
Tests: removed dead code
Tests: added function to test if list items match
Tests: use dummy cache backend

Let's prevent from shooting ourselves in the foot.
Permissions: extend `view` permissions with defaults

Ensures access rights will be extended with those corresponding to the
default users (nobody, default), thus the fully-explicit nature of the
`view` permission is now more relaxed.

Fixes #3344.