Building website is not deterministic

[BD103]

Determinism is a property that allows you to reproduce the exact same website given the exact same files. This would be useful when comparing the effect of changes, since you could run:

$ zola build --output-dir public-old
$ git switch new-branch
$ zola build --output-dir public-new
$ diff --recursive public-old public-new

This currently does not work because random whitespace is inserted by Zola every build, without ever changing the files. To reproduce, run the above command but without switching to a different branch.

The main reason I wanted this is because it could help me review the changes made in #862.