clear_edge_all() for nav_menu_item updates

[rubas]

Changes @ /wp-admin/nav-menus.php trigger for every term purge_term and action_clean_term_cache and for every item action_clean_post_cache and purge_post_with_related.

There is not a lot we can do about it in the current setup, but we can at least make sure that it clears the whole cache as this most likely impacts the whole site.

(Alternative: Find a way to add the relevant menu term to every page and use them instead)

[danielbachhuber]

Closing in favor of further discussion #115