Handle revisionable entities better

Problem/Motivation

Currently $entity instanceof RevisionableInterface is used to check if an entity is revisionable to perform appropriate revision-specific tasks. But this leads to trouble, because some entity types (e.g. terms) implment that interface, but aren't revisionable and thus throw errors upon saving panelizer displays.

Proposed resolution

• Use $entity->getEntityType()->isRevisionable() instead of $entity instanceof RevisionableInterface to determine if revision-specific operations have to be performed

Remaining tasks

• Create a patch to fix this issue

User interface changes

API changes

Data model changes