Trying to pass the current request down to the service so that it can
create full urls instead of paths makes the API messy so use a model
based on ActiveSupport::CurrentAttributes to provide the current host
to services that need it (primarily the disk service).
This fixes following warnings:
```
test/models/variant_test.rb:11: warning: ambiguous first argument; put parentheses or a space even after `/' operator
lib/active_storage/attached/macros.rb:63: warning: instance variable @active_storage_attached_highlights not initialized
lib/active_storage/attached/macros.rb:25: warning: instance variable @active_storage_attached_avatar not initialized
```
