Allow html in nav menu item descriptions

[schonert]

Atm. the wp menu builder strips all html from the wp_nav_menu_description. In case the description needs to be multiple lines, it requires a hardfix - which prevents possible updates.

Removing the esc_html() from from the menu builder, and adding it to the walker, would give us as developers the ability to control this behavior.

[celloexpressions]

Due to the scaling issues with nav menus (#14134, etc.), and because not allowing html will probably result in generally smaller descriptions, I don't think we should do this.

[obenland]

Tags are stripped through a filter (r14826), so not having them stripped anymore is merely a matter of unhooking that function.

I agree with celloexpressions, we should keep them stripped by default.