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/views/upload_media_assets/index.html.erb

@@ -0,0 +1,16 @@
+<div id="c-upload-media-assets">
+  <div id="a-index">
+    <div class="flex items-center mb-2">
+      <h1 class="flex-1">My Uploads</h1>
+
+      <span>
+        <strong>Gallery</strong> |
+        <%= link_to "Table", uploads_path(mode: "table") %>
+      </span>
+    </div>
+
+    <%= render "upload_media_assets/gallery", upload_media_assets: @upload_media_assets %>
+
+    <%= numbered_paginator(@upload_media_assets) %>
+  </div>
+</div>