Minify HTML for f-droid.org and GitLab pages builds

Closed username-removed-25042 requested to merge pserwylo/fdroid-website:minify-html into master

Takes approximately 2-10 times longer, so don't run on other builds. Reduces size from 305MiB - 240MiB which seems like a good win. It might seem trivial for total bandwidth, but for each individual page load the user should notice fast pageloads.

Note that there are other HTML minifiers around, but this was so trivial easy to enable that I'm happy to take the slow build time over a theoretical better minifier which may or may not outperform this, but which would require much more configuration ad scripting.