Fix the sidebar on the /posts index page sometimes being blank. This
could happen when either the related tag calculation was too slow and
timed out, or when Reporbooru was unavailable and we couldn't fetch the
list of popular tags.
In the tag list would otherwise be blank, we fall back to frequent tags
(the most common tags on the current page of results).
Also change it so that if Reportbooru is unconfigured, we fail
gracefully by returning blank results instead of failing with an
exception. This is so we can still view the popular searches and missed
searches pages during testing (even though they'll be blank).
* Combine MissedSearchService, PostViewCountService, and
PopularSearchService into single ReportbooruService class.
* Use Danbooru::Http for these services instead of HTTParty.
* 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.
