Increase font size on extra large screens

Nowadays there are many users with a screen of 1920px (or higher) width
but who don't change their font-size preferences and keep them at 16px,
which is the default in most browsers.

On pages which use the whole window width, that results in unreasonably
long lines which are very hard to read. That's the main reason why many
sites (including CONSUL) use rules like `max-width: 75rem` for the body
or other elements.

However, on extra large screens this causes the content to be in the
middle of the screen while most of the screen is empty, and the text
might also be too small for that resolution, making it harder to read.

There are a few approaches to solve this problem.

Using just viewport units like `font-size: calc(4vw + 4vh + 2vmin);` is
indeed responsive, but it's got an important flaw: it ignores user
preferences, and so the font size will be the same for users who prefer
a 16px size and for users who prefer a 32px size.

Using `calc(1em + 0.2vw)` or similar might make the text too big on
small screens.

Finally, using `max(1em, 1vw)` would work in a similar way to what we're
doing, but zooming in and out would only work when the viewport width is
less than 100em (at that moment, 1em == 1vw).

So we're taking an approach where zooming in and out always works at
least to a certain degree (due to the 0.25em part) and the font size is
increased gradually when we reach the point where the viewport width is
bigger than 100em. We're using 0.25em since it will be exactly 4px with
the default browser configuration, and so calculations are easier than
with (for example) 0.3em.

Disclaimer: I've tested this feature on several devices with different
screen sizes and resolutions, but I must admit there might be cases I'm
unaware of where there are side effects for certain combinations of
screen size/resolution/dpi. In general, though, the side effects should
be similar to what happens when users increase the font size in their
browser's preferences.

Note we're using `Max` instead of `max` because Sass can't handle the
CSS `max` function, as mentioned in commit cdfa23fc6.

app/assets/stylesheets/layout.scss

@@ -33,6 +33,10 @@
   @include normal-selection;
 }
 
+html {
+  font-size: calc(0.25em + Max(0.75em, 0.75vw));
+}
+
 html,
 body {
   height: 100%;