We now have uniform avatar rendering for the profile, messages and
chatboxes.
By rendering as canvas, we can avoid stretching the image.
In the process I also moved the ChatBoxViews collection into its own
plugin `converse-chatboxviews` and placed the AvatarAware views there.
fixes#1157
- Render images as thumbnails
- Use the image.html template when rendering images from pasted URLs
- Update message and spoiler markup to render avatars
- Use the default avatar as fallback when user doesn't have one
- Instead of 'me' render own name or JID
