improve the "my addons" listing (bug 750906)

(moved from [zamboni/pull/1788](https://github.com/mozilla/zamboni/pull/1788))

bug [750906](https://bugzilla.mozilla.org/show_bug.cgi?id=750906)

This PR is based on the PR #1752, as it's the follow-up of bug 750904.

![improve_addons](https://f.cloud.github.com/assets/167767/2398332/8a255130-a9f4-11e3-8aed-640384f2c162.png)