settings: Add setting for SYSTEM_BOT_REALM.

This fixes some subtle JavaScript exceptions we've been getting in
zulipchat.com, caused by the system bot realm there not being "zulip"
interacting with get_cross_realm_users.

zerver/lib/actions.py

@@ -3889,7 +3889,7 @@ def get_cross_realm_dicts():
     # type: () -> List[Dict[str, Any]]
     users = bulk_get_users(list(get_cross_realm_emails()), None,
                            base_query=UserProfile.objects.filter(
-                               realm__string_id="zulip")).values()
+                               realm__string_id=settings.SYSTEM_BOT_REALM)).values()
     return [{'email': user.email,
              'user_id': user.id,
              'is_admin': user.is_realm_admin,
@@ -3899,7 +3899,7 @@ def get_cross_realm_dicts():
             # Important: We filter here, is addition to in
             # `base_query`, because of how bulk_get_users shares its
             # cache with other UserProfile caches.
-            if user.realm.string_id == 'zulip']
+            if user.realm.string_id == settings.SYSTEM_BOT_REALM]
 
 def do_send_confirmation_email(invitee, referrer, body):
     # type: (PreregistrationUser, UserProfile, Optional[str]) -> None

zerver/lib/export.py

@@ -585,7 +585,7 @@ def fetch_user_profile(response: TableData, config: Config, context: Context) ->
 def fetch_user_profile_cross_realm(response: TableData, config: Config, context: Context) -> None:
     realm = context['realm']
 
-    if realm.string_id == "zulip":
+    if realm.string_id == settings.SYSTEM_BOT_REALM:
         response['zerver_userprofile_crossrealm'] = []
     else:
         response['zerver_userprofile_crossrealm'] = [dict(email=x.email, id=x.id) for x in [

zproject/settings.py

@@ -225,6 +225,10 @@ DEFAULT_SETTINGS.update({
     'FEEDBACK_BOT': 'feedback@zulip.com',
     'FEEDBACK_BOT_NAME': 'Zulip Feedback Bot',
     'FEEDBACK_STREAM': None,
+    # SYSTEM_BOT_REALM would be a constant always set to 'zulip',
+    # except that it isn't that on zulipchat.com.  We will likely do a
+    # migration and eliminate this parameter in the future.
+    'SYSTEM_BOT_REALM': 'zulip',
 
     # Structurally, we will probably eventually merge
     # analytics into part of the main server, rather