Source attributes in output template (#45)

* Updated download options to take a media item; allowed for specifying custom output path template options

* Fixed bug where indexing job wouldn't run for the first time

* Renamed friendly_name to custom_name

* Added new options to default template and UI

* Improved explainer UI

lib/pinchflat_web/controllers/media_items/media_item_html/show.html.heex

@@ -17,7 +17,7 @@
           <section>
             <strong>Source:</strong>
             <.inline_link href={~p"/sources/#{@media_item.source_id}"}>
-              <%= @media_item.source.friendly_name %>
+              <%= @media_item.source.custom_name %>
             </.inline_link>
           </section>