Add background gallery to menu

[fitztrev]

On prod, when you select Picture background, there's a selection gallery, but not on local.

prod	local

[fitztrev]

env.net.assetBaseUrl value is http://localhost:8080 which is only accessible via the host machine. When the lila container requests that URL, it's not making it to the nginx container.

This is a hardcoded fix to the Dasher controller that gets it working and helps explain the issue:

   private lazy val galleryJson = env.memo.cacheApi.unit[Option[JsValue]]:
     _.refreshAfterWrite(1.minute)
       .buildAsyncFuture: _ =>
-        ws.url(s"${env.net.assetBaseUrl}/assets/lifat/background/gallery.json")
+        ws.url(s"http://nginx/assets/lifat/background/gallery.json")