Have anon users follow default external link setting

> Security analysis: This sends another SiteSetting to the client - default_external_links_in_new_tab. This is a boolean setting, so information disclosed is limited. Additionally, it is the default for a setting that users are allowed to change. This does not reveal much other than something that might be a personal preference of the site owner, and not anything about the site itself. Sending this site setting has a clear purpose and no real risk.

Is there a better place to put this, other than in `discourse.js`?