Ignore upper/lowercase differences in email when syncing external groups

Whether the Google Sheet has the non-lowercase email address or the ID
Broker database has the non-lowercase email, this will now correctly
sync the external groups (rather than removing them on every other run).

application/common/models/User.php

@@ -1052,17 +1052,24 @@ public static function updateUsersExternalGroups(
             return $errors;
         }
 
+        $desiredExternalGroupsByLowercaseUserEmail = [];
+        foreach ($desiredExternalGroupsByUserEmail as $email => $groups) {
+            $desiredExternalGroupsByLowercaseUserEmail[mb_strtolower($email)] = $groups;
+        }
+        unset($desiredExternalGroupsByUserEmail);
+
         $emailAddressesOfCurrentMatches = self::listUsersWithExternalGroupWith($appPrefix);
 
         // Indicate that users not in the "desired" list should not have any
         // such external groups.
         foreach ($emailAddressesOfCurrentMatches as $email) {
-            if (! array_key_exists($email, $desiredExternalGroupsByUserEmail)) {
-                $desiredExternalGroupsByUserEmail[$email] = '';
+            $lowercaseEmail = mb_strtolower($email);
+            if (! array_key_exists($lowercaseEmail, $desiredExternalGroupsByLowercaseUserEmail)) {
+                $desiredExternalGroupsByLowercaseUserEmail[$lowercaseEmail] = '';
             }
         }
 
-        foreach ($desiredExternalGroupsByUserEmail as $email => $groupsForPrefix) {
+        foreach ($desiredExternalGroupsByLowercaseUserEmail as $email => $groupsForPrefix) {
             $user = User::findByEmail($email);
             if ($user === null) {
                 $errors[] = 'No user found for email address ' . json_encode($email);

application/features/groups-external-sync.feature

@@ -121,4 +121,20 @@ Feature: Syncing a specific app-prefix of external groups with an external list
         | email                  | groups                    |
         | john_smith@example.org | ext-wiki-one,ext-wiki-two |
 
+  Scenario: Properly handle mismatched casing (uppercase/lowercase) in an email address
+    Given the following users exist, with these external groups:
+        | email                  | groups       |
+        | john_smith@example.org | ext-wiki-one |
+        | Jane_Doe@example.org   | ext-wiki-two |
+      And the "ext-wiki" external groups list is the following:
+        | email                  | groups       |
+        | John_smith@example.org | ext-wiki-one |
+        | jane_doe@example.org   | ext-wiki-two |
+    When I sync the list of "ext-wiki" external groups
+    Then there should not have been any sync errors
+      And the following users should have the following external groups:
+        | email                  | groups       |
+        | john_smith@example.org | ext-wiki-one |
+        | Jane_Doe@example.org   | ext-wiki-two |
+
   # Scenario: Send 1 notification email if sync finds group(s) for a user not in this IDP