#4 add user manager interface

[Charles Lepple]

Adding users is easy; deleting them isn't.

It would be nice to have a GUI for adding users. This
would provide some help on how the email and PayPal
settings work.

We can't just delete all records for a given user -
that would screw up the balances.

The deletion process could be two-stage. The first
stage just hides the existence of the user, or grays
out their name. Then, once the user is no longer listed
anywhere (after doing the "balance forward"
reconciliation), they can be safely purged.