users: drop id_to_name, name_to_id caching.

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.

app/views/artist_versions/_standard_listing.html.erb

@@ -37,7 +37,7 @@
           </td>
           <td>
             <%= link_to_user artist_version.updater %>
-            <%= link_to "»", artist_versions_path(search: { updater_name: artist_version.updater_name }) %>
+            <%= link_to "»", artist_versions_path(search: { updater_name: artist_version.updater.name }) %>
             <p>
               <%= compact_time(artist_version.updated_at) %>
               <% if CurrentUser.is_moderator? %>