Add nest extension hook to nav menus

[briandominick]

I have not really thought this one through, but it came up as I was documenting the nav menus, so I wanted to pin it on the board.

I believe all of the manually configurable menus (site nav in the header, subject nav in the left sidebar, and company nav in the footer) could accommodate additional nesting.

I have run a quick test on Navgoco (subject nav), and it can handle at least another tier of nesting. I am just not sure I want to support that.

The Bootstrap-driven site nav can definitely handle more nesting. See https://bootstrap-menu.com/detail-multilevel.html. This can look rather nice, but is it ever necessary? I haven't been asked to implement anything that hardcore for a site menu, but it wouldn't be hard.

But I might have to put my foot down before trying this in the footer. There is plenty of room down there, though stream-of-consciousness I guess I can fathom wanting to have lots of links in the footer but keeping it sparse at first glance from a design perspective.

Anyway, the issue is placing an extension hook at the deepest point of nesting, in case you want to continue the iteration in your own templates.