Display images in notification comment detail

Adds support for displaying images in the notification comment detail view. ImageSpans are added to the comment content at the index provided in the `Note` object.

_Note: There's currently a bug in the Simperium model that will return duplicate indices for images if they are next to each other in the HTML markup. Should be fixed server-side soon._

Before:
![screen shot 2014-09-25 at 5 01 41 pm](https://cloud.githubusercontent.com/assets/789137/4424340/199e8dfa-459e-11e4-8813-b8d191eb5151.png)

After:
![screen shot 2014-09-26 at 9 35 59 am](https://cloud.githubusercontent.com/assets/789137/4424341/2100fdbc-459e-11e4-996d-5c117e67daa9.png)