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.

test/functional/uploads_controller_test.rb

@@ -148,7 +148,7 @@ class UploadsControllerTest < ActionDispatch::IntegrationTest
       context "with search parameters" do
         should "render" do
           search_params = {
-            uploader_name: @upload.uploader_name,
+            uploader_name: @upload.uploader.name,
             source_matches: @upload.source,
             rating: @upload.rating,
             has_post: "yes",