* 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.
Change the title of the post index page to look like this:
"Danbooru: Anime Image Board" (for the front page)
"Kantai Collection Art | Danbooru" (for a tag search)
Change the meta description of the front page to look like this:
Danbooru is the original anime image 'booru. Find over 3.75 million
anime pictures categorized by over 100 million tags.
Change the meta description for a tag search to look like this:
Find over 37,168 Azur Lane images on Danbooru. Azur Lane (碧蓝航线)
(벽람항로) is a Chinese shipgirl-themed side-scrolling shoot 'em up
mobile game developed by Shanghai Manjuu and Xiamen Yongshi...
* Factor out common sidebar layout template.
* Convert wiki pages and posts to use this template.
* Add data-layout attribute to <body> element indicating the current layout.
This causes the browser to prefetch the next page in a search, which may
make browsing seem faster to the end user, but it effectively doubles
the load on the server.
Avoid some queries used in wiki page excerpts:
* Only try to load the artist if the tag is an artist tag.
* Avoid using `exists?` queries for wiki pages.
* Bugfix: don't show wiki excerpts for deleted wikis.
Refactor tag_list_html, split_tag_list_html, and inline_tag_list_html to
take the `show_extra_links` and `current_query` options explicitly,
rather than implicitly relying on CurrentUser or taking `params[:tags]`
from the template.
