Auto-filter on new PPA-index page

The new PPA-index page (not yet landed but available at lp:~soyuz-team/launchpad/3-0-ppa-index-merge-branch) has a simple filter for the packages table, which should default to filtering by the distroseries determined by the user-agent (bug 425548).

It would be nice if the Filter button was removed via JS and the filtering updated using the onchange event so that users can simply select an alternate distroseries without having to then click Filter.