[upstream] HTML5 transparent content model support - block-level links in CKEditor

CKEditor at now don't allow by default insert <div> elements inside <a>, here https://github.com/ckeditor/ckeditor-dev/issues/514 this problem is accepted and provided a solution via redefine DTD in config:

// Simply redefine DTD like this:
CKEDITOR.dtd['a']['div'] = 1;
CKEDITOR.dtd['a']['p'] = 1;
CKEDITOR.dtd['a']['i'] = 1;
CKEDITOR.dtd['a']['span'] = 1;

Before this will be fixed in CKEditor, Drupal users already want to use HTML5 transparent content model support.

So will be good to apply provided solution in Drupal core.