apps/versions/feeds: add an ?all option to the format:rss page to display the entire 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.
apologies, I misread your code. mine has the exact same issues.

AMO has its own custom [Paginator](https://github.com/mozilla/olympia/blob/7199c1e/apps/amo/helpers.py#L198) that you can use. but it may be worth looking into [Django's built-in Pagination](https://docs.djangoproject.com/en/1.7/topics/pagination/) (which I believe was added in Django 1.6; AMO is on Django 1.6.5).