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/core/organizations.rs

@@ -540,18 +540,19 @@ fn send_invite(org_id: String, data: JsonUpcase<InviteData>, headers: AdminHeade
     }
 
     for email in data.Emails.iter() {
+        let email = email.to_lowercase();
         let mut user_org_status = if CONFIG.mail_enabled() {
             UserOrgStatus::Invited as i32
         } else {
             UserOrgStatus::Accepted as i32 // Automatically mark user as accepted if no email invites
         };
-        let user = match User::find_by_mail(email, &conn) {
+        let user = match User::find_by_mail(&email, &conn) {
             None => {
                 if !CONFIG.invitations_allowed() {
                     err!(format!("User does not exist: {}", email))
                 }
 
-                if !CONFIG.is_email_domain_allowed(email) {
+                if !CONFIG.is_email_domain_allowed(&email) {
                     err!("Email domain not eligible for invitations")
                 }
 
@@ -601,7 +602,7 @@ fn send_invite(org_id: String, data: JsonUpcase<InviteData>, headers: AdminHeade
             };
 
             mail::send_invite(
-                email,
+                &email,
                 &user.uuid,
                 Some(org_id.clone()),
                 Some(new_user.uuid),