Make single plugin mode reponsiveness more dynamic

[caseycesari]

Overview

Makes two changes to how single plugin mode mobile/responsive adjustments are handled:

• Replace max-device-width with max-width. Using max-width instead of max-device-width allows the site to adapt to smaller viewports on laptops and desktops instead of just mobile devices.
• Rework adjustment of single plugin mode for mobile. Instead of adjusting the header once at app initialization, adjust the header as the app is resized. This allows for the app to feel more responsive when resizing the browser on desktop.

Connects #1135

Demo

Testing Instructions

• Turn on single plugin mode.
• Visit the site in the browser, and resize the window.
• Verify the header adjusts to the browser size by 1) switching between an expanded and collapsed search control, and 2) hiding the title and subtitle when the search is focused in mobile mode.
• Visit the site on a mobile device, and verify that it still adjusts correctly.

[kellyi]

Taking a look at this!

[caseycesari]

Thanks!