posts index: clean up limit:<n> / order:random parsing (#2894)

* Move the limit:<n> / order:random metatag parsing from the controller
  to the post set.

* Introduce `Tag.has_metatag?` and use it to parse these metatags
  instead of using a regex (#2894).

app/models/tag.rb

@@ -445,6 +445,13 @@ class Tag < ApplicationRecord
       tag.include?("*")
     end
 
+    def has_metatag?(tags, *metatags)
+      return nil if tags.blank?
+
+      tags = scan_query(tags.to_str) if tags.respond_to?(:to_str)
+      tags.grep(/\A#{Regexp.union(metatags.map(&:to_s))}:(.+)\z/i) { $1 }.first
+    end
+
     def parse_query(query, options = {})
       q = {}