Use spaces everywhere

[Forty-Bot]

Python uses four-space indents. However, all the other files use eight-width tabs. This works OK for C, which tends not to be highly nested, but for HTML it can get really out of hand. Often HTML has six or more levels of indentation, not counting jinja directives which I typically don't indent. I think switching to four spaces is probably best, but two should be considered as well. This will unfortunately make git blame rather useless for converted files.