Adjust mobile single web app "About" page z-index to prevent obscuring the hamburger menu

[kellyi]

Overview

This PR adjusts the z-index of the "About" element in mobile-mobile-single-web-app mode so that it will no longer obscure the hamburger menu when the latter's open.

Connects #1013

Demo

Testing Instructions

• get this branch
• update the singlePluginMode object in region.json to set "active": true, then rebuild the app in Visual Studio and serve it
• visit the site in Chrome in the normal browser
• verify that the "About" link in the header displays an "About" element and that it's not obscured beneath anything
• verify that the hamburger icon on the right header still functions as usual
• turn on the mobile emulator in Chrome devtools, then hard refresh the page
• click the hamburger icon to verify that the menu appears
• click the "About" link to verify that that element gets displayed
• with the "About" element displayed, click on the hamburger icon again and verify that the selection menu isn't hidden beneath the "About" element

[kellyi]

Oops, I had accidentally set this to target master, but I changed it to target develop.

[kellyi]

Thanks for reviewing! The lorem ipsum + placeholder image is correct for the default branch; it's something that can be configured in the region.json file.