[Enhancement] Custom media lifecycle scripts (#219)

* Namespaced notification modules under lifecycle

* Added a JSON encoder for all the main model types

* Added startup task to create user script file

* Hook up user script event to media download

* Hooked up media deletion user script

* Added jq to docker deps

* Updated README

lib/pinchflat/lifecycle/user_scripts/user_script_command_runner.ex

@@ -0,0 +1,10 @@
+defmodule Pinchflat.Lifecycle.UserScripts.UserScriptCommandRunner do
+  @moduledoc """
+  A behaviour for running custom user scripts on certain events.
+
+  Used so we can implement Mox for testing without actually running the
+  user's command.
+  """
+
+  @callback run(atom(), map()) :: :ok | {:error, binary()}
+end