Add username suggestion based on patron name

Add username suggestion based on patron name

Add support for suggesting a username based on the patron's first
and last name.

Currently, the scheme for suggestion is a lowercased first initial +
last name, with any characters other than [a-z] removed.

No effort is made to suggest an available username, but if the
suggested username is already in use, the standard "username in use"
message is displayed next to the username field.

Staff can initiate the username suggestion by clicking the new
button next to the username field, and usernames are automatically
suggested when loading a staged user.

Signed-off-by: Jeff Godin <jgodin@tadl.org>