Add rel=nofollow to various internal search links to prevent Google from
attempting to crawl things like `<tag> status:deleted` or `approver:<name>`
searches.
* Convert notices from helpers to partials.
* Eliminate PostSets::PostRelationship class in favor of post_sets/posts template.
* Eliminate COUNT(*) queries when calculating the number of child posts.
* Eliminate redundant parent load and parent exists queries.