On the posts show page, in the favorites list, show favorites according
to the order they were added to the favorites table, rather than the
order they were added to the posts's fav_string.
On most posts these should be the same, but on old posts they may be
slightly different. The IDs of the first few hundred thousand favorites
don't appear to be in chronological order. Probably the original
favorite IDs were lost and recreated by a database move at some point in
Danbooru's history. The fav_string is also inconsistent with the
favorites table in some places (one contains favorites that aren't
contained by the other), which also throws off the order.
Partially addresses #4562 by eliminating one place where we depended on
the fav_string.
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