EmptyPane model

[#47372055]
Associate EmptyPane to Page

[#47372055]
AddEmptyPanes migration

[#47372055]
SgPermissions for EmptyPane

[#47372055]
Controller actions for EmptyPane

[#47372055]
"New" link for EmptyPane

[#47372055]
Redirect to page after destroying EmptyPane

[#47372055]
Serialization/copying support for EmptyPane

[#47372055]