Add infrastructure policy

Draft based on James's and my conversation last Friday.

See the Zulip conversation about this MR. Comments also welcome here, of course.

(I wish there were a way to just embed Zulip threads right here into GitLab, instead of having a foreign link)

merged

mentioned in commit 8f5b5c20

Okay, merged. Further improvement is both possible and welcome, but I didn't see anything in the discussion thread that would preclude merging what we have so far.