Switch to WebIO for all communication tasks

This release updates all of MeshCat's internal communication to use WebIO.jl. That means that, in addition to working in Jupyter and in a standalone browser tab, MeshCat will also work inside Juno and in a standalone Blink.jl window. Jupyter integration is now better, and it should be possible to use MeshCat even on a remote Jupyter server like JuliaBox.

This change does, however, break compatibility with the old ZMQ interface. That means that communication between Julia and Python MeshCat sessions is not currently possible.