[OPS] Automation around purging

[rgbkrk]

Right now, when we make a deploy or update the image it likely doesn't look very live because most pages are cached by Fastly. Independently of this repo, we should trigger the purge against fastly for nbviewer.

[yuvipanda]

https://docs.fastly.com/api/purge has references on how to purge. It allows us to purge everything, which we could possibly do on each deploy (not sure how much additional load it would put on servers?).

At Wikimedia, cached HTML (for logged out users, in article views) is set to cache for 30 days only, producing a nice and gradual turnover without too much cache issues.