Refactor fav:<name> and ordfav:<name> searches to use the favorites
table instead of the posts.fav_string.
This may be slower for fav:<name> searches. The fav_string effectively
treats favorites like secret tags on the post, so fav:<name> searches
were effectively the same as tag searches. Now they do a subquery on the
favorites table, which may not perform as well for things like multiple
fav:<name> metatags or negated fav:<name> metatags.
For ordfav:<name> searches, this may be faster. ordfav: searches had a
tag match clause (`tag_index @@ 'fav:123'`) in addition to a join on the
favs table. This was redundant, and in some cases it inhibited the query
planner from choosing a more optimal plan.
Partially addresses #4652 by eliminating another place where we depended
on the fav_string.
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