Limit which blocks an editor can add to a content area (AllowedTypes)

Description

The allowed types attribute now supports interfaces and also provides the option to restrict a subset of the allowed types. For example, if a developer wants to allow all page types except container pages then they would specify PageData as the allowed type and ContainerPage as the restricted type.

The changes to the allowed type attribute are reflected in the user interface where drag and drop continues to function as previously with the added support for restricted types. The user interface has been extended to support allowed and restricted types when creating new blocks via the "create a new block" link in the content area. Clicking the "create a new block" link in the content area will now only show types that are configured as being allowed in the content creation screen.