Add html extension to files in src/public_html/HTML/

This is to make syntax highlighting work. I personally use nano for editing. It will highlight syntax but doesn't detect what the syntax is. It uses the filename extension. Therefore, the extension must be correct or syntax highlighting doesn't work. It won't hurt anything, and could actually help in other situations, so it should be done.

Will, of course, require some alteration of the inclusion mechanism in src/include/GetDivs.php