[Enhancement] Added ability to detect when files have been deleted (#399)

* Added function for updating a media item's filepaths on-disk

* Added placeholder action to source page

* Turned the file sync into a job and properly hooked it up to the controller

lib/pinchflat_web/router.ex

@@ -40,6 +40,7 @@ defmodule PinchflatWeb.Router do
       post "/force_redownload", Sources.SourceController, :force_redownload
       post "/force_index", Sources.SourceController, :force_index
       post "/force_metadata_refresh", Sources.SourceController, :force_metadata_refresh
+      post "/sync_files_on_disk", Sources.SourceController, :sync_files_on_disk
 
       resources "/media", MediaItems.MediaItemController, only: [:show, :edit, :update, :delete] do
         post "/force_download", MediaItems.MediaItemController, :force_download