Server side configuration

Handle all configuration, including all per-user options, notebook lists, and notebook attributes, as specific, highly-granular RCS key/value pairs, rather than glomming them into the `config.json` relic.

Introduces an upgrade script `scripts/upgradeRCS1_0.R` which moves keys around in RCS (using whatever backend is configured).

Fixes #324 potential RCS conflict with a user named `notebook` (by moving notebook metadata to `.notebook`.
Fixes #351, removing `config.json`
Fixes #307, making sure different sessions don't clobber each others notebooks (by virtue of much finer granularity changes to the configuration).
Introduces a Most Recently Used list for a better notebook to open when removing the current one for #235 
Makes life easier for #298 per-user options.