Disable template digesting for Active Storage controllers

Error entries appear in the log when we request ActiveStorage
controllers (`ActiveStorage::Representations::ProxyController#show`,
`ActiveStorage::Blobs::ProxyController#show`).

These entries look like: "Couldn't find template for digesting:
active_storage/representations/proxy/show".

These controllers use the method
`ActionController::ConditionalGet#http_cache_forever`,
and therefore `ActionController::ConditionalGet#combine_etags` method,
and therefore `ActionController::EtagWithTemplateDigest` module via
`etaggers` array.

`ActionController::EtagWithTemplateDigest` module requires a template
(view).

We have no views in ActiveStorage, so `EtagWithTemplateDigest` is
now turned off in ActiveStorage controllers by
`etag_with_template_digest` class attribute.

activestorage/app/controllers/active_storage/base_controller.rb

@@ -6,6 +6,8 @@ class ActiveStorage::BaseController < ActionController::Base
 
   protect_from_forgery with: :exception
 
+  self.etag_with_template_digest = false
+
   private
     def stream(blob)
       blob.download do |chunk|