Reduce nav element duplication by consolidating the Main and Sticky Navs

[benlk]

The basic proposal:

• Remove the Main Nav
• Put the Sticky Nav in the Main Nav slot, with all its resizing-related code
• If the sticky flag is checked, use https://caniuse.com/#feat=css-sticky to pin it to the top of the viewport rather than JS.

Inspired by https://pudding.cool/process/scrollytelling-sticky/

Related tickets:

• stuff re: sticky nav https://github.com/INN/largo/issues/1627
• duplicate social media button IDs https://github.com/INN/largo/issues/1781
• text node within header image link: https://github.com/INN/largo/issues/1783
• Header Widget Area doesn't show on mobile: https://github.com/INN/largo/issues/1650
• consider whether we'd want to use Flexbox to evenly space these items: https://github.com/INN/largo/issues/1608

[MirandaEcho]

Unify the sticky and Main navigations to reduce CSS/JS overhead

[benlk]

IIRC we did this in CIJN and Citylimits.

[benlk]

Also, add actions all over the place here so that things can be added to the nav without requiring duplicating partials.