Gravatar hover preview

This PR is to get an MVP for this feature out there, then we can expand it further as we want to.

User list hover:
![](https://jabbrlive.blob.core.windows.net/jabbr-uploads/clipboard_9000.png)

Message list hover:
![](https://jabbrlive.blob.core.windows.net/jabbr-uploads/clipboard_e1ef.png)

Fixes #516