Multiple notebooks open at once?

[switham]

Two ways this can happen:

1. multiple "tabs" or something open on the local user's browser (less likely on Android, right?)
2. allowing users over the net who are just using a browser.

Trickier to support multiple open notebooks on a phone without multiple tabs. Could be done as a single page flipping its contents by AJAX. "ajaxplexing"

If it is done, the list of notebooks should do the right thing with currently open notebooks:

• can't go into an already-open one unless ajaxplexing.
• an external browser user could go into an open notebook by cloning it first. (sharing a live open notebook is not what this issue is about)
• show if a code block is currently still executing in any.

If we allow external users, that means we need sessions (issue ??)