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.
nope, just do it like this:

https://github.com/mozilla/zamboni/blob/master/mkt/reviewers/tests/test_views.py#L2679-L2682

the nice part about mocking is you're guaranteed to not leak changes. if `feeds` is imported from other tests and you had forgotten to revert the `feeds.PER_PAGE` change in the `tearDown` method, we'd run into intermittent test failures.

your code works, and your tests are quite good, but it's just something worth considering - since we try to use `mock` wherever possible.