posts: add is: and has: metatags.

Add the following metatags:

* is:parent
* is:child
* is:safe
* is:questionable
* is:explicit
* is:sfw (same as -rating:q,e)
* is:nsfw (same as rating:q,e)
* is:active
* is:deleted
* is:pending
* is:flagged
* is:appealed
* is:banned
* is:modqueue
* is:unmoderated
* is:jpg
* is:png
* is:gif
* is:mp4
* is:webm
* is:swf
* is:zip
* has:parent
* has:children
* has:source
* has:appeals
* has:flags
* has:replacements
* has:comments
* has:commentary
* has:notes
* has:pools

All of these searches were already possible with other metatags, but these might be more convenient.

app/logical/autocomplete_service.rb

@@ -13,12 +13,14 @@ class AutocompleteService
   POST_STATUSES = %w[active deleted pending flagged appealed banned modqueue unmoderated]
 
   STATIC_METATAGS = {
+    is: %w[parent child sfw nsfw] + POST_STATUSES + MediaAsset::FILE_TYPES + Post::RATINGS.values.map(&:downcase),
+    has: %w[parent children source appeals flags replacements comments commentary notes pools],
     status: %w[any] + POST_STATUSES,
     child: %w[any none] + POST_STATUSES,
     parent: %w[any none] + POST_STATUSES,
     rating: Post::RATINGS.values.map(&:downcase),
     embedded: %w[true false],
-    filetype: %w[jpg png gif swf zip webm mp4],
+    filetype: MediaAsset::FILE_TYPES,
     commentary: %w[true false translated untranslated],
     disapproved: PostDisapproval::REASONS,
     order: PostQueryBuilder::ORDER_METATAGS