posts: add more free metatags.

The following metatags no longer count against the tag search limit:

* is
* id
* date
* age
* filesize
* filetype
* parent
* child
* md5
* width
* height
* duration
* mpixels
* ratio
* score
* upvotes
* downvotes
* favcount
* embedded
* tagcount
* pixiv_id
* pixiv

These are mostly metatags that have to do with properties of the post
itself. Other metatags still count because they involve things like
subqueries or joins, or they're more tag-like in function.

test/unit/post_query_builder_test.rb

@@ -1353,7 +1353,7 @@ class PostQueryBuilderTest < ActiveSupport::TestCase
       post1 = create(:post, rating: "s")
 
       assert_raise(PostQuery::TagLimitError) do
-        PostQuery.search("a b c rating:s width:10 height:10 user:bob", tag_limit: 5)
+        PostQuery.search("a b c user:bob fav:bob pool:disgustingly_adorable", tag_limit: 5)
       end
     end
 
@@ -1363,6 +1363,7 @@ class PostQueryBuilderTest < ActiveSupport::TestCase
       assert_tag_match([post1], "aaa bbb rating:s")
       assert_tag_match([post1], "aaa bbb status:active")
       assert_tag_match([post1], "aaa bbb limit:20")
+      assert_tag_match([post1], "aaa bbb filesize:<100mb width:<10000 height:<10000 limit:20")
     end
 
     should "succeed for exclusive tag searches with no other tag" do