apps/versions/feeds: add pagination support for RSS version history

This fixes bug https://bugzilla.mozilla.org/show_bug.cgi?id=1070766 by adding a ?all option to the Version History RSS feed, that displays the entire history.

I had thought about enabling pagination but decided this was simpler, since even with pagination there is no easy way (for a crawler) to figure out "the last page", which is what I really need to fix https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=762310.
Could you also add the `rel="first"` (with `self.page.start_index()`, or maybe just with `self.feed['feed_url']`) and `rel="last"` (with `self.page.end_index()`) while you're at it? :bouquet: