Fix propagation of avatar URLs

Our current code doesn't update the avatar of a Gittip account when it takes over another account. This results in the avatar not being updated after connecting an elsewhere account (because connecting is implemented as a take over). Moreover, the avatar wasn't being updated after disconnecting an elsewhere account either.

This PR fixes all that.

Thanks to @seanlinsley (https://github.com/gittip/www.gittip.com/pull/2314#issuecomment-41454031) and @YenaHong ([IRC](https://botbot.me/freenode/gittip/msg/13860356/)).