Security: Harden Papiea against various web vulnerabilities

[joshua-berry-ntnx]

We should make sure we are following the best practices defined by Express for deploying secure web services:

https://expressjs.com/en/advanced/best-practice-security.html

In particular, we should probably look at using the Helmet middleware to set appropriate headers to prevent CSRF and similar things—not that we are likely to be vulnerable to many of them in our specific situation, but it takes very little time and might save a big headache later.