R1:301fed1b43a4

Revise administrative workflow for user creation

Summary:
- When an administrator creates a user, provide an option to send a welcome
email. Right now this workflow kind of dead-ends.
- Prevent administrators from changing the "System Agent" flag. If they can
change it, they can grab another user's certificate and then act as them. This
is a vaguely weaker security policy than is exhibited elsewhere in the
application. Instead, make user accounts immutably normal users or system agents
at creation time.