posts: eliminate pool_id, favgroup_id url params.

Replace the `pool_id` and `favgroup_id` url params:

    https://danbooru.donmai.us/posts/123?pool_id=456
    https://danbooru.donmai.us/posts/123?favgroup_id=456

with the `q` param:

    https://danbooru.donmai.us/posts/123?q=pool:456
    https://danbooru.donmai.us/posts/123?q=favgroup:456

app/logical/post_query_builder.rb

@@ -949,8 +949,8 @@ class PostQueryBuilder
       metatags.select { |term| term.name.in?(names.map(&:to_s)) }
     end
 
-    def find_metatag(metatag)
-      select_metatags(metatag).first.try(:value)
+    def find_metatag(*metatags)
+      select_metatags(*metatags).first.try(:value)
     end
 
     def has_metatag?(*metatag_names)