Improve metadata storage (#37)

* Updated config path to specify metadata storage path

* Removed metadata column from DB, replacing it with filepath columns

* Updated app to store compressed metadata; automatically download thumbnails

* Ensured metadata is deleted when other files are deleted

* Updated docs

* Added inets to application start so http calls work in prod

lib/pinchflat/http/http_behaviour.ex

@@ -0,0 +1,8 @@
+defmodule Pinchflat.HTTP.HTTPBehaviour do
+  @moduledoc """
+  This module defines the behaviour for HTTP clients. Literally just
+  so I can use Mox to create an HTTP mock
+  """
+
+  @callback get(String.t(), Keyword.t(), Keyword.t()) :: {:ok, String.t()} | {:error, String.t()}
+end