Convert all update handling to use updates

[chriskrycho]

Instead of just as simple date, always make it a list:

updates:
  - at: <date>

Then update all updated: <date> to be updates: [<date>].

[chriskrycho]

It turns out this is a slightly more complicated change than I thought, because it's interacts interestingly with the idea of the "started" tag I created. Maybe it works if, and only if, I maintain two separate stacks of history: pre-“publication” and post-“publication”. That itself implies an interesting distinction, and runs right into some of the issues I note in Feeds Are Not Fit for Gardening. 🤔 More to consider here. (Also relevant for how I choose to encode this in my next-gen site builder.