* Add ability to search /post_versions by added tags, removed tags, or
changed tags (added or removed).
* Add 'History' link to the sidebar of the /posts index. This is a
shortcut for a /post_versions search of the current tag.
Changes:
* Drop Users.id_to_name.
* Don't cache Users.name_to_id.
* Replace calls to name_to_id with find_by_name when possible.
* Don't autodefine creator_name in belongs_to_creator.
* Don't autodefine updater_name in belongs_to_updater.
* Instead manually define creator_name / updater_name only on models that need
to return these fields in the api.
id_to_name was cached to reduce the impact of N+1 query patterns in
certain places, especially in api responses that return creator_name /
updater_name fields. But it still meant we were doing N calls to
memcache. Using `includes` to prefetch users avoids this N+1 pattern.
name_to_id had no need be cached, it was never used in any performance-
sensitive contexts.
Avoiding caching also avoids the need to keep these caches consistent.
* Allow every controller to take the `search[id]` param.
* Parse the `search[id]` param the same way that the `id:<N>` metatag is
parsed. So `search[id]=1,2,3`, `search[id]=<42`, `search[id]=1..10`, for
example, are all accepted.
