Fix bug 972533: Enhancements to admin handling of deleted pages.

This adds a new manager to Document, which does not perform any
filtering based on deleted status, and uses that manager for the
admin; this ensures the admin always shows all Documents currently in
the database.

Building on that, the admin filters now include one to show only
deleted Documents.

The admin actions for Document are also updated:

* A new "purge" action is added, to bulk-purge deleted Documents.

* A new "undelete" action is added, to bulk-undelete deleted
  Documents.

* The built-in Django "delete" action is removed, since we want people
  using the standard deletion UI (and because the admin delete action
  bypasses the model delete() method, which is not what we want for
  deleting Documents).
Render document URLs with the already defined Document.get_absolute_url method instead of inline.
fix bug #1002371 - don't lose the query parameters when switching languages.
Fix bug 1008828: Show correct locale/slug in page move error.

The root of the problem was a child page move failing after the
child's locale/slug had already been changed in-memory; in that case
the new locale/slug would end up in the error message instead of the
original.

Now the original locale/slug get stashed away prior to attempting the
child move, and if the child move fails they get inserted in the error
message to ensure the error points to the correct URL.
Make selection of links into Quick Links less greedy for toggleable lists.
Fixed anchor is null issue pointed out by @darkwing
Use latest kuma-lib to fix an issue with Django and hash_ring.
Also install newest version of Django on Travis.
Merge pull request #2347 from ubernostrum/deleted-pages-972533

Fix bug 972533: Enhancements to admin handling of deleted pages.
Merge pull request #2394 from riverspirit/promote-mdn-fix-1

Fixed anchor is null issue pointed out by @darkwing
Merge pull request #2393 from JeremiePat/master

Quick Links glitch
fix bug 1010024 - Ajaxify subscribe link
Merge pull request #2392 from darkwing/subscribe-1010024

fix bug 1010024 - Ajaxify subscribe link
Remove all compat-* functions in Stylus

The compat-* functions were used to override styles from the old design.
The functions used !important to do this. By removing the functions and
the associated use of !important, our stylesheets will be more
maintainable and reliable. This also reduces the size of compiled CSS by
2.04KB.

Some bugs were fixed just by removing these functions. At the same time,
some bugs were revealed by removing these functions. (For example,
errors related to selector specificity suddenly manifested as
user-facing bugs when !important was removed.)

Newly-manifesting bugs like these were searched for and corrected, but
it is possible that some remain. If we find any more, we can correct
them with later commits.
hides the submenu on focusout
Merge pull request #2402 from icaaq/hide_submenu_on_focusout

hides the submenu on focusout
fix bug 1010030 - Ensure spacing for PRE elements
Merge pull request #2391 from darkwing/pre-spacing-1010030

fix bug 1010030 - Ensure spacing for PRE elements
Merge pull request #2389 from ubernostrum/page-move-error-1002040

Fix bug 1008828: Show correct locale/slug in page move error.
Merge pull request #2401 from openjck/remove-compat

Remove all compat-* functions in Stylus