Notification Rich Text Support

#### Notifications: Rich Text Support

Now Text and Comment notifications have support for inline images. We've implemented a simple component, *RichTextView*, based on TextKit.

The original goal was to wire *RichTextView*'s delegate to asynchronously download the images. However, that breaks if the image size is originally unknown: the user may / may not specify the embedded image size, beforehand.

For that reason, a simple helper class, *NotificationMediaDownloader* was implemented: this class receives a list of URL's, and will hit a completion callback whenever the assets are downloaded.

Duplicate downloads are prevented, and a simple cache has been implemented.

Closes #2661 

/cc @aerych + @dan may i bother you with a code review?. 
Thanks in advance!
(I think) if this code matures enough, we'll be switching soon to TextKit!