LP#1246859: Improve username flow for staged users

Staged users in most cases have a username that consists of a UUID.

Rather than leave the UUID as their username or require staff to
clear the field and replace it with a barcode or other value, it
would be better to remove this username when loading the staged user
in the user editor.

This commit clears the username when loading a stage user if the
username looks like a UUID.

If a barcode has been staged for the loaded staged user, the
username will be populated with that value.

If there is no staged barcode, the username will remain blank and
will be populated using the standard method after staff enters a
value in the barcode field.

Signed-off-by: Jeff Godin <jgodin@tadl.org>
Signed-off-by: Ben Shum <bshum@biblio.org>