Remove some #try from attributes_for_render

* By the time we're here, we're at a recognised host, which necessarily
  implies an associated site and org
* Some things may still not be there, but those things are limited to:
  * A mapping (implies 404)
  * A TNA timestamp (implies nothing very much, as a default will be generated)

**Note** some extra stubbing is required. To minimise this, use `as_null_object` (not doing so requires an extra half-dozen stubs and increases test brittleness). We care about actual calls to `site`, so we can't avoid being explicit about what will be called there. Add those calls.