pfVerticalNavigation: on mobile, move the masthead utility items to the vertical navigation

[dtaylor113]

Description

Added a mobileOnly property to nav. item objects; when 'true' items only display in mobile mode. Addresses: #745

Ex: The Help and User vert. nav items only appear when in mobile (< 768px) mode, and are hidden when not in mobile mode:

Note the masthead utility items (Help & User dropdowns) continue to be hidden at mobile; this PR allows app devs to mimic them 'moving' to the vert nav menu.

@alexkieling

PR Checklist

• [x] Screenshots are attached (if there are visual changes in the UI)
• [x] A Designer (@beanh66) is assigned as a reviewer (if there are visual changes in the UI)

[dtaylor113]

Hi @jeff-phillips-18, I believe I have addressed your comments, except the one about the Help-at-mobile menu, IMO I think leaving off the 'About' menu item represents a truer @mobile menu.

[dtaylor113]

Might want to address #746 while you are working here...

I'd prefer to do that in a follow up pr, thanks :-)