Tweaks to website generation - now with more files

[David A. Wheeler]

I've made further tweaks to the website generator.
It should be using every .mm data in the set.mm repo,
and various .html / .raw.html files are getting used.

For the moment the us2.metamath.org site is getting
updated daily, and the us.metamath.org site is also getting
updated daily (after us2). That lets me see if things are working.
I think we might want to keep it that way, though a slower update
cadence might also make sense, but let's get things working first :-).

--- David A. Wheeler