Add support for nested menu

[IshmeetSingh06]

Apparently active_material does not support nested submenus for the active admin, Is this by choice or we have not updated the package to support it?

For example, when I try to create a menu item with nested submenu, it messes up the layout:

As shown in the image the submenu is overlapping the main menu and some of the menu items are being hidden
Nested menu support was added a while back in active admin activeadmin/activeadmin#5994
If you remove the styling provided by active_material and just use the core styling by active admin it works fine:

[IshmeetSingh06]

Hi @dkniffin any plans on having this implemented?

[dkniffin]

@IshmeetSingh06 I'd welcome a PR for adding support for this, but I'm not sure when I'll get to adding support for it myself.