WL 1894 Signup muddles usernames and user ids

This fixes some of the confusing behaviour in the sign up tool where the ui
refers to user IDs but this corresponds to the enterprise ID as opposed to
the Oxford LDAP username (which under the hood is known as display id).

This addresses:

 +  **WL-1892** sakai ID shown instead of username (in list of instructors).
 + **WL-2588** signup only accepts email addresses when assigning attendees.
 + In **WL-1894** the specific behaviour of showing E IDs in the warning message.