apps/versions/feeds: support pagination (bug 1070766)

We also add rel links to other pages, as per RFC 5005. This includes a tweak
to the semantics of the rel="self" link to refer to the *current* page; the
rel="first" link adopts its former semantics referring to the *first* page.
Merge pull request #301 from infinity0/master

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