this is not used anymore:
* not logged in users are redirected to the login page
* logged in users see the normal 404 page

closes #6652

- move mobile icons to mobile-folder, so they aren't included in the
  icons-sprite
- move logos to branding/logos, and create a logos-sprite without the
  big other branding-images
- delete duplicate images

closes #6004

body was rendered in the head

This allows us to reuse any CSS we have, unify
their look and unify their look with the regular
page design.

This works by instantiating ActionView and rendering
templates in a rake task.

Inspired by the errgent gem.

to specify why users got a 404, and possibly leaking information while doing this

See discussion in #3717 for rationale on not implementing specific HTTP codes
This solves around #3717 and #3728

Adjusting travis breakage

and tries to access a show page of a non-public post