Fix checking for allowed tag hierarchy for tables. The way, the check is
currently done (not transitive), the tag hierarchy for table related tags
(td, th, tr) has to be set to 'div' to enable inserting tables into
editables that are themselves contained in non-editable tables.
Merge pull request #1134 from npomaroli/hotfix-rt57823

Fix checking for allowed tag hierarchy for tables. The way, the check is