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.

config/routes.rb

@@ -252,7 +252,9 @@ Rails.application.routes.draw do
       get :batch
       get :image_proxy
     end
+    resources :upload_media_assets, only: [:show, :index], path: "assets"
   end
+  resources :upload_media_assets, only: [:show, :index]
   resources :users do
     resources :favorites, only: [:index, :create, :destroy]
     resources :favorite_groups, controller: "favorite_groups", only: [:index], as: "favorite_groups"