admin: Restore missing volumes tab
admin: Add admin-only Http404 exception
admin: Add middleware for 404 exceptions

Add a custom middleware that will get activated explicitly when an
AdminHttp404 exception is raised.

This middleware will get the message of the exception and use it to
render a 404 page (/admin/admin_404.html) with that message in it.
admin: Use AdminHttp404 in any view necessary

Replace Http404 exception with AdminHttp404 exception in most of the
views. Also, write a descriptive message in each exception that will be
shown in 404 page.

Fix apyrgio/synnefo#242
admin: Add tests for 404
Merge branch 'feature-admin-404' into feature-admin