Clean up site navigaiton submenu behavior

[admturner]

Is your suggestion related to a problem? Please describe.

Would like to find a more graceful solution to #80. Currently it uses some extra JS to toggle the aria-expanded attribute when a toggle is a current page ancestor. This works, but not as smoothly as it should.

Describe alternatives you have considered.

Hopefully the solution will come from Core, but at the very least would like to intercept the behavior at the WP Core script level instead of attempting to override it.