Add mechanism to have workspaces skip processing entity types

Problem/Motivation

Developers should have the option to allow specified entities to be ignored by workspaces.

For example, I specifically wanted to allow the manipulation of file and crop entities in a non-default workspace.

Proposed resolution

Move the shouldSkipPreOperations method out of EntityOperations and into the WorkspaceManager service and make it public.

Add a protected array of entity_type ids to WorkspaceManager called $skiplist. This matches the pattern used by blacklist. Developers that wish to add entity_type ids to the $skiplist can do so by decorating the workspaces.manager service.