Reduce public API methods from ModalHelper

Before this PR, including `bh` in an app would include more methods
than necessary for modals: methods like `dialog_class` and
`modal_body` that should only be accessed privately.

This PR extracts those private methods into a new Modal class
that includes all the business logic to render elements and edit
attributes (e.g., attach classes), leaving the helper cleaner.
Reduce duplication testing modal helpers