refactor menu item attributes

This commit merges `menu_item_menu_name` into `navigation_menu_name`, as
they both appear to have the same purpose. By doing so, this seems to fix a
previously unreported issue of belongs_to nested resources. Now, the menu
items for those nested resources show up in a unique navigation menu
specific to the parent resource.
Merge pull request #1987 from Daxter/refactor/1967-menu-updates

Refactor [1967] Menu Updates