batches: add filtered repo batch changes page and badge on repo page

Created by: courier-new

Closes #20309 (closed).

Follow-up to #22744 that introduces the new repo batch changes page and a badge to get there from a repo's own page!

The badge button thing currently sits off to the right of the repo title:

@rrhyne have you had a chance to consider if we want to convert this to one of the tab button things below the repo title, and just use the open stat badge, like you suggested during our impromptu design review last week?

The filtered repo page shows the list of batch changes with one or more changesets created in the given repo, or the getting started details when no batch changes have been made yet.