A11y: Mobile nav button should be reachable via keyboard

[bacitracin]

Describe the task

When the screen is narrower and the mobile nav hamburger button pops up, it is not reachable or operable via keyboard navigation. Since it's not reachable by keyboard, screenreaders are affected as well.

This ticket is the FIRST part of making the mobile nav more accessible.

1. You should be able to TAB to the hamburger menu (probably need to add tabIndex=-1),
2. You should be able to press ENTER or SPACE and the dropdown opens.

Resources

Total expected behavior of the menu:
(THIS ticket):
You should be able to TAB to the hamburger menu, press ENTER or SPACE and the dropdown opens.

(Second ticket)
Then you can use the UP or DOWN arrow keys to navigate the items (with focus state visible). Press ENTER or SPACE to choose one of the items. Page will change.

(Third ticket)
While the dropdown is open pressing ESC or TAB will close the dropdown and focus will go back to the hamburger button. The option is NOT selected. Page does not change.

See this example for most of the desired behavior (except for the behavior while the dropdown is open and pressing TAB.) https://www.w3.org/WAI/ARIA/apg/patterns/disclosure/examples/disclosure-navigation/#mythical-page-content