* Move various search parser helper methods (`has_metatag?`,
`is_single_tag?` et al) from PostSets and the Tag model to
PostQueryBuilder.
* Fix various minor bugs stemming from trying to check if a search query
contains certain metatags using regexes or other adhoc techniques.
Add a curated posts page at /explore/posts/curated. Curated posts are
the most favorited posts by contributor-level users (users with
unlimited upload permissions).
Also add an order:curated tag using for use in regular searches.
This was used to discourage crawlers from crawling certain pages we
didn't want them to crawl, primarily post searches.
Remove because there are better ways to control crawling. Some of these
links weren't even visible to crawlers anyway. This lets us be
consistent about only applying rel="nofollow" to external links.
* Add ability to search /post_versions by added tags, removed tags, or
changed tags (added or removed).
* Add 'History' link to the sidebar of the /posts index. This is a
shortcut for a /post_versions search of the current tag.
