use html5 history api to update url

but it crashes if you press back/forward too quickly
closes #197, #250, #305
artifacts
disabling the history stack due to concurrency issues

i think the concurrency issues (if you click 'back' repeatedly)
are fixable, but i'd like to start this feature out with just replacing
history since it's already helpful.
Merge branch 'develop' into feature/notebook-url
Merge branch 'feature/notebook-url' into develop
Merge branch 'develop' of https://github.com/att/rcloud into develop