fix(avatars): expose option to change your profile image

This adds a "Change" link to the settings page and makes the avatar clickable, too.
Fixes #1845.
Fixes #1787.
![screen shot 2014-11-19 at 4 48 09 pm](https://cloud.githubusercontent.com/assets/3903/5117700/e3cbac32-700c-11e4-9b18-222dd5e7a108.png)
After writing this, I realize - Ghostery blocks Gravatar - is this something we should attempt to detect?