Add fastly cache purge to deployment step

[Keno]

Right now, changes to the website don't really get reflected quickly, because we're fronted by fastly. One can manually curl -X PURGE it to fix that, but it's not a great solution. Ideally, the deployment step, but build both the old and the new version, diff it and then purge any pages that changed. If that's too complicated, we can probably just do a global fastly purge also.