Move constants to config, update string formatting

Not sure if this is the Elixir-y way to do this, but seems more logical
than hardcoding values such as redis connection.

Also went through and improved how string formatting was performed
throughout the app. Rather than "combining" <> "strings" this way, I'm
now just doing "#${variable}#{formatting}", which looks a lot cleaner.

lib/privacy_revolver/application.ex

@@ -1,4 +1,5 @@
 defmodule PrivacyRevolver.Application do
+  @redis_conn Application.fetch_env!(:privacy_revolver, :redis_conn)
   @moduledoc false
 
   use Application
@@ -7,7 +8,7 @@ defmodule PrivacyRevolver.Application do
   def start(_type, _args) do
     children = [
       Plug.Cowboy.child_spec(scheme: :http, plug: PrivacyRevolver.Router, options: [port: 4001]),
-      {Redix, {"redis://localhost:6379", [name: :redix]}}
+      {Redix, {@redis_conn, [name: :redix]}}
     ]
 
     opts = [strategy: :one_for_one, name: PrivacyRevolver.Supervisor]

lib/privacy_revolver/router.ex

@@ -1,9 +1,15 @@
 defmodule PrivacyRevolver.Router do
+  @fallback_str Application.fetch_env!(:privacy_revolver, :fallback_str)
+
   use Plug.Router
 
   plug :match
   plug :dispatch
 
+  get "/" do
+    send_resp(conn, 200, "")
+  end
+
   get "/ping" do
     # Useful for app healthcheck
     {:ok, resp} = Redix.command(:redix, ["PING"])
@@ -11,19 +17,30 @@ defmodule PrivacyRevolver.Router do
   end
 
   get "/:service/*glob" do
-    full_path = "/" <> Enum.join(glob, "/")
-    {:ok, instances} = Redix.command(:redix, ["LRANGE", service, "0", "-1"])
+    path = Enum.join(glob, "/")
+    {:ok, instances} = Redix.command(
+      :redix,
+      ["LRANGE", service, "0", "-1"]
+    )
 
-    # Either pick a random available instance, or fall back to the default one
+    # Either pick a random available instance, 
+    # or fall back to the default one
     instance = if Enum.count(instances) > 0 do
       Enum.random(instances)
     else
-      Redix.command(:redix, ["GET", service <> "-fallback"])
+      {:ok, result} = Redix.command(
+        :redix,
+        ["GET", "#{service}#{@fallback_str}"]
+      )
+      result
     end
 
     # Redirect to the available instance
     conn |>
     Plug.Conn.resp(:found, "") |>
-    Plug.Conn.put_resp_header("location", instance <> full_path)
+    Plug.Conn.put_resp_header(
+      "location",
+      "#{instance}/#{path}"
+    )
   end
 end