As a Developer, I want navigation to be responsive for desktop/mobile

[acobster]

Acceptance Criteria

• [ ] Main navigation turns into a hamburger menu at a certain breakpoint (q. for Reena)
• [ ] On mobile, menu items are revealed when the hamburger icon is clicked, and hidden again on subsequent click
• [ ] On desktop, sub-menus are toggled on parent mouse hover
• [ ] On mobile, sub-menus are toggled on parent mouse click/tap
• [ ] Menu hamburger icon turns into an "X" when the menu is open on mobile
• [ ] Main navigation items collapse into hamburger menu and are hidden by default on mobile
• [ ] Confirm that standard breakpoints apply