LDAP email address downcasing

Fixes #2960

In the event we cannot match an LDAP user by DN we attempt to find an identity by email address and then update the DN. In this case the identity is matched by email address. 

If the user's email address in LDAP has an upper case character we cannot find a match in the GitLab database. GitLab downcases emails before the user object is saved.

This merge request downcases the email from LDAP before we lookup by email. I also added a test to prevent a regression.

See merge request !1550