register: Activate existing mirror dummy only if on same realm.

The one thing this bit of logic is used for is to decide whether
there's an existing user which is a mirror dummy that we should
activate.  This change causes us to ignore such an existing user if
it's on some other realm, and go straight into `do_create_user`.

zerver/views/registration.py

@@ -14,7 +14,7 @@ from django.core import validators
 from zerver.context_processors import get_realm_from_request
 from zerver.models import UserProfile, Realm, Stream, MultiuseInvite, \
     name_changes_disabled, email_to_username, email_allowed_for_realm, \
-    get_realm, get_user_profile_by_email, get_default_stream_groups
+    get_realm, get_user, get_default_stream_groups
 from zerver.lib.send_email import send_email, FromAddress
 from zerver.lib.events import do_events_register
 from zerver.lib.actions import do_change_password, do_change_full_name, do_change_is_admin, \
@@ -168,10 +168,13 @@ def accounts_register(request: HttpRequest) -> HttpResponse:
         if 'timezone' in request.POST and request.POST['timezone'] in get_all_timezones():
             timezone = request.POST['timezone']
 
+        if not realm_creation:
             try:
-            existing_user_profile = get_user_profile_by_email(email)
+                existing_user_profile = get_user(email, realm)
             except UserProfile.DoesNotExist:
                 existing_user_profile = None
+        else:
+            existing_user_profile = None
 
         return_data = {}  # type: Dict[str, bool]
         if ldap_auth_enabled(realm):