The aim here is to make the theme work respectably on mobile devices
with only modest changes. Providing different layouts at multiple
breakpoints is beyond the scope of this change.
The changes here are:
1. `base.html`: Add a `<meta name="viewport"` element
2. `main.css`:
* Use "max-width" instead of "width"
* Set "line-height" on the banner and adjust vertical spacing to match
* Remove fixed height on the nav bar and force it to contain its
child elements
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