Create new config to control visibility on entity types

Problem/Motivation

Currently in handlebars_entity_view_alter() we hardcoded the list of entity types where the handlebars templates should be included.

Proposed resolution

This needs to be configurable via Admin form.

Remaining tasks

- Add logic in the admin form to choose the visibility form a list of options (checkboxes)
- The list of options should be automatically populated from available entity types

Additionally we should consider a checkbox to 'Show on every page', that would make the list disabled when this option is selected.