Add Form Controller to preview view modes

Problem/Motivation

Follow up from ✨ Add view tab + standard template for block content Needs work

From @larowlan

add a form to the controller to support previewing in various view modes like Node preview has.

Steps to reproduce

TBD

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

TBD

Release notes snippet

TBD