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/comment.rb

@@ -17,8 +17,8 @@ class Comment < ApplicationRecord
   end
   mentionable(
     :message_field => :body, 
-    :title => ->(user_name) {"#{creator_name} mentioned you in a comment on post ##{post_id}"},
-    :body => ->(user_name) {"@#{creator_name} mentioned you in a \"comment\":/posts/#{post_id}#comment-#{id} on post ##{post_id}:\n\n[quote]\n#{DText.excerpt(body, "@"+user_name)}\n[/quote]\n"},
+    :title => ->(user_name) {"#{creator.name} mentioned you in a comment on post ##{post_id}"},
+    :body => ->(user_name) {"@#{creator.name} mentioned you in a \"comment\":/posts/#{post_id}#comment-#{id} on post ##{post_id}:\n\n[quote]\n#{DText.excerpt(body, "@"+user_name)}\n[/quote]\n"},
   )
 
   module SearchMethods
@@ -183,6 +183,14 @@ class Comment < ApplicationRecord
     super + [:creator_name, :updater_name]
   end
 
+  def creator_name
+    creator.name
+  end
+
+  def updater_name
+    updater.name
+  end
+
   def delete!
     update(is_deleted: true)
   end
@@ -192,6 +200,6 @@ class Comment < ApplicationRecord
   end
 
   def quoted_response
-    DText.quote(body, creator_name)
+    DText.quote(body, creator.name)
   end
 end