Integration tests with Faircamp

[audiodude]

At some point, I decided to checkout a specific Faircamp tag when building the site, so that random changes to the dev branch don't break us. That was a good idea.

However, now because of Rust upgrades and library upgrades, I am forced to upgrade the Faircamp version (previous version won't build).

I am going to do some manual testing, but I should really have integration tests that check that for certain directory layouts that Rainfall is likely to produce, Faircamp not only runs without error but also produces a reasonable site.