Generating static index.html files takes a long time

[toolness]

I've noticed that generating the static index.html files takes a very long time: about 14 seconds on my fairly new-ish desktop, and upwards of 30 seconds on my 2010 Macbook Air. Removing the static file generation process entirely actually makes the rest of the build process very quick, so it's definitely not webpack or anything else that's slowing things down.

Aside from making things slow, this also has a negative impact on the webserver's auto-reload functionality: often when making a code change, webpack will reflect the change into the dist directory almost instantly, which results in a page refresh. At this point I'll start interacting with the page, only for it to reload yet again about 12 seconds later because the index files have finally regenerated, which triggers another reload.

Even though we'll likely be moving away from a static site in #585, this issue is still relevant since we don't want server-side rendered code to take eons to generate.

[toolness]

It turned out the culprit was actually gulp-prettify, which was taking 12 of those 13 seconds! Switching to gulp-html-prettify sped things up immensely.