Allow bypassing app router with FARSIDE_NO_ROUTER

Setting the aforementioned env var skips creation of the app router,
which is useful for running update.exs when the main app is already
running (otherwise there's a port conflict).

lib/farside/application.ex

@@ -1,4 +1,5 @@
 defmodule Farside.Application do
+  @farside_port Application.fetch_env!(:farside, :port)
   @redis_conn Application.fetch_env!(:farside, :redis_conn)
   @moduledoc false
 
@@ -6,12 +7,22 @@ defmodule Farside.Application do
 
   @impl true
   def start(_type, _args) do
-    children = [
-      Plug.Cowboy.child_spec(scheme: :http, plug: Farside.Router, options: [port: 4001]),
-      {Redix, {@redis_conn, [name: :redix]}},
+    plug_children = [
+      Plug.Cowboy.child_spec(
+        scheme: :http,
+        plug: Farside.Router,
+        options: [
+          port: @farside_port
+        ]
+      ),
       {PlugAttack.Storage.Ets, name: Farside.Throttle.Storage, clean_period: 60_000}
     ]
 
+    children = [
+      {Redix, {@redis_conn, [name: :redix]}} |
+      System.get_env("FARSIDE_NO_ROUTER") && [] || plug_children
+    ]
+
     opts = [strategy: :one_for_one, name: Farside.Supervisor]
     Supervisor.start_link(children, opts)
   end