Improve display of connected accounts and avatars.

Improve the display of Connected Accounts and avatars, as discussed in #2154.

Some notes:

* If an account is not linked it'll show the generic avatar instead of nothing (#2196).
* Things without images: these seem to be special cased anyway (Bitcoin, Balanced).
* Although I advocated for having the avatar be the smaller one, after trying that, the avatars proved to look horrible, so I made them the larger of the two pictures.

Closes #2154.

![gittip-issue-2154-fix-screenshot](https://cloud.githubusercontent.com/assets/39698/2653833/ec2e6ae4-bfc8-11e3-8f25-f6254a1e13b3.png)