improve the devhub versions page (bug 750914)

(moved from [zamboni/pull/1795](https://github.com/mozilla/zamboni/pull/1795))

bug [750914](https://bugzilla.mozilla.org/show_bug.cgi?id=750914)


Not being aware of the various statuses (on addons/versions/files) and their
consequences, I'd love a thorough review and tests on this.

Eg: a version is seen as disabled (and faded out) if all its files' statuses
are disabled, is that correct?

This PR is based on the PR #1788, which is based on the PR #1752. So only the last commit is relevant to this PR.

![improve_versions](https://f.cloud.github.com/assets/167767/2398353/b296572c-a9f4-11e3-8ac5-b683773d454c.png)