`ActiveStorage::DiskController#show` generates a 404 Not Found response when
the requested file is missing from the disk service. It previously raised
`Errno::ENOENT`.
Since ActiveStorage::Blob::Representable unifies the idea of previews and
variants under one roof as representation, we may as well have the
controllers follow suit.
Thus ActiveStorage::RepresenationsController enters the fray. I've copied
the old tests for both previews and variants and unified those as well.
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
```
If an AWS bucket name includes a `.` (e.g. `bucket.name`), then the canonical
URL for an object will start with "https://s3.amazonaws.com/bucket.name/"
and not with "https://bucket.name.s3.amazonaws.com/".
The URL tests have now been separated into two separate asserts, to ensure
that both the "s3.amazonaws.com" and the "bucket.name" components are included,
but not specifically in that order.
