Add streaming_api_base_url to /api/v1/instance (#3556)

* Add streaming_api_base_url to /api/v1/instance

* Adjust spaces

* nested view

* re-adjust spaces

app/views/api/v1/instances/show.rabl

@@ -5,3 +5,6 @@ node(:title)       { Setting.site_title }
 node(:description) { Setting.site_description }
 node(:email)       { Setting.site_contact_email }
 node(:version)     { Mastodon::Version.to_s }
+node :urls do
+    { :streaming_api => Rails.configuration.x.streaming_api_base_url }
+end