Fix issue when using uppercase chars in emails

In the case when SMTP is disabled and.
when inviting new users either via the admin interface or into an
organization and using uppercase letters, this would fail for those
users to be able to register since the checks which were done are
case-sensitive and never matched.

This PR fixes that issue by ensuring everything is lowercase.
Fixes #1963

src/api/admin.rs

@@ -269,7 +269,7 @@ fn invite_user(data: Json<InviteData>, _token: AdminToken, conn: DbConn) -> Json
         if CONFIG.mail_enabled() {
             mail::send_invite(&user.email, &user.uuid, None, None, &CONFIG.invitation_org_name(), None)?;
         } else {
-            let invitation = Invitation::new(data.email);
+            let invitation = Invitation::new(user.email.clone());
             invitation.save(&conn)?;
         }