Add viewport meta tag with `width=device-width`

When I load the page on my Android phone, it's too wide and zoomed far
out. Hoping this will help. It should set viewport width to "the width
of the screen in CSS pixels at a scale of 100%." Source:
https://developer.mozilla.org/en-US/docs/Mozilla/Mobile/Viewport_meta_tag
Merge branch 'mobile-friendly'