Activate size calculations of download sized on website

Activate size calculations of download sized on website

This deals with the aftermath of issue 5822 by enabling the size
calculations while building the website in order to reinstate the
download sizes, now correct, that have disappeared in the wake of this
issue.

I HAVE NO CLUE WHAT I AM DOING HERE, so it would be good if someone
with a better grasp of the website setup checked this patch out.
Regtesting it will be useless as the requisite parts are only
activated at website build time.

In case of failure, our website may refuse to build/update.  It is not
clear what will happen with the stable version docs under which
conditions.

Apart from the update possibly stopping, the consequences should be
benign: HTML comments are already replacing the size estimates.  This
looks inconspicuous and can only get better™.

If the updates get held up, reverting this patch is the simplest
temporary fix.  The way I understand things, the website should remain
offline: the worst that happens is that updates stop until this patch
is reverted.