[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/notifications/apprise_command_runner.ex

@@ -0,0 +1,12 @@
+defmodule Pinchflat.Lifecycle.Notifications.AppriseCommandRunner do
+  @moduledoc """
+  A behaviour for running CLI commands against a notification backend (apprise).
+
+  Used so we can implement Mox for testing without actually running the
+  apprise command.
+  """
+
+  @callback run(binary(), keyword()) :: :ok | {:error, binary()}
+  @callback run(List.t(), keyword()) :: :ok | {:error, binary()}
+  @callback version() :: {:ok, binary()} | {:error, binary()}
+end