[Enhancement] Allow redownloading of files for existing media items (#239)

* Added ability to specify overwrite behaviour when downloading media

* Added helper for redownloading media items

* renamed media redownload worker to disambiguate it from similarly named methods

* Added new redownload option to source actions dropdown

* Refactored MediaQuery to use a __using__ macro

* docs

lib/pinchflat/media/media_item.ex

@@ -4,6 +4,8 @@ defmodule Pinchflat.Media.MediaItem do
   """
 
   use Ecto.Schema
+  use Pinchflat.Media.MediaQuery
+
   import Ecto.Changeset
   import Pinchflat.Utils.ChangesetUtils
 
@@ -12,7 +14,6 @@ defmodule Pinchflat.Media.MediaItem do
   alias Pinchflat.Sources
   alias Pinchflat.Tasks.Task
   alias Pinchflat.Sources.Source
-  alias Pinchflat.Media.MediaQuery
   alias Pinchflat.Metadata.MediaMetadata
   alias Pinchflat.Media.MediaItemsSearchIndex