Improve L10n dashboard filters

Improvements to filters and order options of the l10n dashboard.

* Fix order by and allow more orderings by locale and by English documents
* Make "update needed" the default view. There is no reason to show all locale docs. We just want the outdated ones here.
* Introduce a "missing parent" filter where locale teams are able to find missing associations to the English documents.

(The third filter is the "in progress" filter, which does not seem to work how it should be. Needs more review - a new bug probably)