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/models/application_record.rb

@@ -274,10 +274,6 @@ class ApplicationRecord < ActiveRecord::Base
               rec.creator_ip_addr = CurrentUser.ip_addr if rec.respond_to?(:creator_ip_addr=)
             end
           end
-
-          define_method :creator_name do
-            User.id_to_name(creator_id)
-          end
         end
       end
 
@@ -288,10 +284,6 @@ class ApplicationRecord < ActiveRecord::Base
             rec.updater_id = CurrentUser.id
             rec.updater_ip_addr = CurrentUser.ip_addr if rec.respond_to?(:updater_ip_addr=)
           end
-
-          define_method :updater_name do
-            User.id_to_name(updater_id)
-          end
         end
       end
     end