Update web fonts

After careful consideration I have made a couple of decisions:

1. Switch the main font from FiraGO to Clear Sans (more below);
2. Stop providing a bespoke monospaced font, for the sake of
   performance.

I have selected Clear Sans because (the first three are also true for
FiraGO):

1. It is metrically similar to widespread system fonts, like Helvetica,
   Arial, Roboto.  The base font size matters because the width of the
   content area is defined in rem (multiples of the base font size ==
   16px).  As such, a smaller font, say, Alegreya Sans, will fit in much
   more characters on the same line.  Conversely, a typeface with wide
   proportions, like DejaVu Sans, will only fit a few characters per
   line.  I generally try to be within the range of 70-90 characters per
   line.  Working with a reliable point of reference is essential.
2. It has a rather dark typographic colour, as opposed to, say, Open
   Sans.  This makes it good for legibility.  It also means that its
   shapes are not too fancy, which contributes to its functional
   purpose.  In a similar vein, its x-height is neither too tall nor too
   short, making it well-balanced overall.
3. It supports Latin and Greek scripts.
4. Its numerals are aligned on a grid, as you would expect from
   a monospaced font.  This is excellent news because it means that I no
   longer need to provide a bespoke mono font for the parts of my site
   that need tabular alignment (such as index pages where one column
   shows the date of the publication and the other the title).