Rename front-page.html to front_page.html

Rename front-page.html to front_page.html

I'm trying to maintain consistency of everything through the code, so a
show_front_page method calls render_html with a template name of front_page
and then uses a template called front_page.html. I have vague hopes that I
may be able to type less this way later, but am refusing to modify stuff as
yet since I'll only end up committing premature generalisation by accident.