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

test/farside_test.exs

@@ -77,4 +77,22 @@ defmodule FarsideTest do
       assert first_redirect != second_redirect
     end)
   end
+
+  test "/https://..." do
+    parent_service = "https://www.youtube.com"
+    parent_path = "watch?v=dQw4w9WgXcQ"
+    conn = test_conn("/#{parent_service}/#{parent_path}")
+
+    redirect = elem(List.last(conn.resp_headers), 1)
+
+    IO.puts("")
+    IO.puts("    /#{parent_service}/#{parent_path}")
+    IO.puts("    redirected to")
+    IO.puts("    #{redirect}")
+
+    assert conn.state == :set
+    assert conn.status == 302
+    assert redirect =~ parent_path
+    assert !(redirect =~ parent_service)
+  end
 end