user_groups: Populate membership audit logs during realm creation.

This tracks user group membership changes when the realm is first set
up, either through an import or not. This happens when we add users to
the system user groups by their roles.

For an imported realm, we do extra handling when the data doesn't include
user groups. This gets audited as well.

zerver/lib/import_realm.py

@@ -1677,3 +1677,15 @@ def add_users_to_system_user_groups(
                 UserGroupMembership(user_profile=user_profile, user_group=full_members_system_group)
             )
     UserGroupMembership.objects.bulk_create(usergroup_memberships)
+    now = timezone_now()
+    RealmAuditLog.objects.bulk_create(
+        RealmAuditLog(
+            realm=realm,
+            modified_user=membership.user_profile,
+            modified_user_group=membership.user_group,
+            event_type=RealmAuditLog.USER_GROUP_DIRECT_USER_MEMBERSHIP_ADDED,
+            event_time=now,
+            acting_user=None,
+        )
+        for membership in usergroup_memberships
+    )