Add support for users

[sanity]

Currently the only way to keep worksheets around is to bookmark the URL - this URL gives a user read-write to the worksheet, and is the only current means of authentication.

To facilitate sharing of worksheets, and collaborative work on extending LastCalc's vocabulary, we need user accounts. I think we should keep them simple (probably something like Reddit's approach).

At the simplest level a user account would simply store a list of the worksheets that a user has read-write access to, which they gain just by visiting the worksheet via it's URL.