remove any previous workspace on loading the viewer

this fixes #997 in a stupid and unsatisfying way
fix botched refactor of tag versioning

many names mismatched :-(
caught and fixed just in time for the release :-)
do not try to augment an error

you will only get worse an error!
manually add fork_of to self-forked notebooks

fixes #946
more evidence that we need to encapsulate the bad things we are
doing to emulate forking one's own notebook, as per #831
view.html load by tag

fixes #1002, other page
allow loading mini.html and shiny.html versions by tag name

fixes #1002 yet some more

also, mini.js was not properly chaining the init & call promises
meaning that it was possible call_notebook could have been called
before the session was fully initialized