Replace external_account_id with slug

The account ID isn't useful on its own since it has to be formatted to be used as part of a URL. Therfore let's return the slug instead.

app/views/identities/_account.json.jbuilder

@@ -1,4 +1,4 @@
 json.cache! account do
-  json.(account, :id, :name, :external_account_id)
+  json.(account, :id, :name, :slug)
   json.created_at account.created_at.utc
 end