WL-1894 Add a method for getting a list of display IDs for an email

Currently the check finds a list of enterprise IDs for an email but they
are less meaningful in our situation than the display IDs (which correspond
to LDAP usernames).
WL-1894 Replace references to `eID` with `displayId`

This commit also renames the langauge string for multiple userids.
WL-1892 Show display id instead of enterprise id in list of instructors
WL-2588 Allow the signup facade to fetch a user by display id
WL-2588 Extend id search to include display names

When adding a participant we currently check for enterprise IDs and
email addresses.
Merge pull request #11 from ox-it/WL-1894

WL 1894 Signup muddles usernames and user ids