Auto select frontend for links to "parent" service

Farside now supports redirecting based on a provided link to a "parent"
service, if such a parent service is supported.

For example, a link such as:

farside.link/https://www.youtube.com/watch?v=dQw4w9WgXcQ

will now redirect to any of the available YouTube related frontends.

This works by matching against a mapping of "parent" service domains
("youtube.com", "reddit.com", etc) to a list of their respective frontend
alternatives (["invidious", "piped"], ["libreddit", "teddit"], etc). A
random element is chosen from this list, and the remainder of Farside's
routing logic proceeds as if the user had chosen the service directly to
begin with.

Closes #37

lib/farside/router.ex

@@ -39,12 +39,27 @@ defmodule Farside.Router do
   end
 
   get "/:service/*glob" do
-    path = Enum.join(glob, "/")
+    service_name = cond do
+      service =~ "http" ->
+        List.first(glob)
+      true ->
+        service
+    end
+
+    path = cond do
+      service_name != service ->
+        Enum.join(Enum.slice(glob, 1..-1), "/")
+      true ->
+        Enum.join(glob, "/")
+    end
+
     instance = cond do
       conn.assigns[:throttle] != nil ->
-        Farside.last_instance(service)
+        Farside.get_service(service_name)
+        |> Farside.last_instance
       true ->
-        Farside.pick_instance(service)
+        Farside.get_service(service_name)
+        |> Farside.pick_instance
     end
 
     params =