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)
To what end would we perform the detection? To provide a more accurate error message?