Support email aliases on LDAP/SAML synched users

A user that is synched from LDAP (via SAML say) has a single canonical address.

However they often have multiple email aliases.  eg

bradbaker@foo, bbaker@foo and brad@foo all represent the canonical user brad.baker@foo

Today there is no email aliases stored.

This is problematic for JIRA Server Desk say on an incoming email ticket because bbaker@foo will not match and existing user and hence we will try to create a new user and onboard them into the system or reject the email ticket outright because we dont know the user.

IM needs the ability to store email aliases so we can look them up and find the right person to associate the ticket with.