NEW: Add SilverStripe\ORM\UnexpectedDataException class.
This change provides more graceful handling of the case where a
ChangeSetItem is referencing a no-longer-existing class.
The new exception, SilverStripe\ORM\UnexpectedDataException, is
intended to be available for throwing whenever we have unexpected data
in the database.
It can be trapped by the relevant UIs and more graceful errors than
HTTP 500s can be provided.
Introducing <Tabs> component based on react-bootstrap
Better support for nested fields in FormBuilder
Tweaks to get FormBuilder working with frameworktest BasicFieldsPage fields
Added exception in FormBuilder when Component is defined but not found
Added check in SingleSelectField for empty value before adding one in
Added temporary workaround for CompositeFields with no name (another story to address the actual problem)
Added asset_preview_height for File image preview, matches the defined CSS max-height
Added documentation to DBFile::PreviewLink() method
