Refactor 'preprocess-*' and 'generate-news' into around-like functions

All preprocess-* functions are now called "around" the original renderer, instead of in separate passes. This approach facilitates passing information between each of the processing steps. The major driver for this change is the need to pass around the original file name, which we need to generate the Edit this page button.

Introduce a way to declare dependencies between pages. That way, we can convert generate-news into a post-processor for news.md, delaying the rendering of index.html until news.md has been generated.

And last but not least: actually add the 'Edit this page' button with a link to our GitLab project!

@dcooper, @mmontone, @vdardel, @tplotnikov: your reviews are highly appreciated! This is quite a big change and I think it's good that we agree on this path forward.

Closes #8 (closed).