[meta] Support all core content entity types

This is a meta issue to track adding support for all of core's content entity types.

In Drupal 7, we supported all of them: node, user, comment and taxonomy term.

In Drupal 8, there are those as well as some new content entity types (message, block_content, shortcut, file, menu_link_content, aggregator_item, aggregator_feed) and some of them are quite unusual, so we may decide not to support some of them if it doesn't make sense.

Child issues will be created for each entity (beyond 'node' which is already supported in Panelizer 3.0-alpha1).