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.
For this use-case, `apps/versions/urls.py` only defines `^format:rss$`. Even if I add random other query strings to it, e.g. with https://addons.mozilla.org/en-US/firefox/addon/tree-style-tab/versions/format:rss?lol&x=y the `rel="self"` link href (and therefore `self.feed['feed_url']`) still points to the url without query strings. So I'm not sure it's worth the effort generalising it to the degree you mention.