uploads: add /upload_media_assets index page.

This page shows each individual file you've uploaded. This is different
from the regular uploads page because files in multi-file uploads are
not grouped together.

app/policies/upload_media_asset_policy.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class UploadMediaAssetPolicy < ApplicationPolicy
+  def show?
+    user.is_admin? || record.upload.uploader_id == user.id
+  end
+end