MM-11229: Fix invite people slash command and add test case. (#9074)

i18n/en.json

@@ -630,6 +630,10 @@
     "id": "api.command_invite.permission.app_error",
     "translation": "You don't have enough permissions to add {{.User}} in {{.Channel}}."
   },
+  {
+    "id": "api.command_invite_people.permission.app_error",
+    "translation": "You don't have permission to invite new users to this server."
+  },
   {
     "id": "api.command_invite.private_channel.app_error",
     "translation": "Could not find the channel {{.Channel}}. Please use the channel handle to identify channels."