Improve localization dashboard performance

This introduces the iDisplayStart param from the ajax table to the view, because we currently try to display *all* documents from the DB in the dashboard table and that's probably why the dashboard fails for large locales and for the "All locales" filter. So, let's just do it like in the revision dashboard.

In addition I'm stealing ideas from [here] (https://github.com/mozilla/kuma/commit/39dd99f487e6c503e5dcddeb331f4121348c44b2) to improve performance.

Didn't test this with many documents on my local box, but HTH :-)