Prepend /u/ for Bibliogram redirects if not present

Bibliogram links require a "/u/" prefix when viewing a user's profile,
which does not match Instagram's URL paradigm. This adds a bit of logic
for Bibliogram links to prepend this prefix if the user is not visiting
a post (indicated by a "/p/" prefix, which Bibliogram and Instagram BOTH
use) or the home page of an instance (indicated by an empty path value).

lib/farside/router.ex

@@ -66,9 +66,11 @@ defmodule Farside.Router do
       conn.assigns[:throttle] != nil ->
         Farside.get_service(service_name)
         |> Farside.last_instance
+        |> Farside.amend_instance(service_name, path)
       true ->
         Farside.get_service(service_name)
         |> Farside.pick_instance
+        |> Farside.amend_instance(service_name, path)
     end
 
     # Redirect to the available instance