[FIX] forum: optimize vote metadata queries

Saves 2 requests per post, aka 20 requests on a full page (89 -> 68):

* don't create a new sudoed env and recordset for each post, create a
  single one so prefetch can work
* don't do unnecessary sudoed access (.parent_id.create_uid without
  reading any field)
* temp variables in template seems to save a few queries, not sure why

There's still a linear growth of requests wrt posts in the
page (single-post page has 56 queries), but it's a bit less terrible.

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