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
Instead of padding the container, pad individual elements that need it.
This enables us to have highlighting (e.g. on hover) that spans the full
width of the controlbox.
updates #1069
