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.