Before this commit, using ActionText and calling `#as_json` on a
non-persisted `ActiveStorage::Blob` raised an error.

This is because `ActionText::Attachable` is included in
`ActiveStorage::Blob` and overrides the `#as_json` method to
expose a global signed id. However, a global signed id can only
be generated on a persisted instance.

This commit fixes the issue by making sure the blob is persisted
before exposing the global signed id.