Set permissions with email addresses

[rpenate]

The Harvard WorldMap implementation of GeoNode allows users to assign layer/map access permissions by email address instead of (in addition to?) existing username. It then creates accounts for addresses that don't already exist and sends an email with their username and temporary password

This is a nice idea and, as we're also considering something similar for our future groups workflow (http://dev.geonode.org/trac/wiki/Groups#a3.ManagerView), we should extend our permissions system (http://dev.geonode.org/trac/ticket/864#comment:4) to allow for email addresses.

Note: This will bring up some issues... Do we allow users to register multiple email addresses to their user account or is it a one-to-one relationship? It would perhaps be useful to allow users to register multiple email addresses, select a primary for notifications, and allow for some method of duplicate account resolution from the user profile (http://dev.geonode.org/trac/wiki/UserProfile) to prevent the proliferation of accounts.