Set publish dates correctly for historical posts

As a follow-up to 3177ae3f, let's actually set these dates correctly.

We can pick these up from the Git commit dates, and makes it more representative of the actual datetime that these posts were published.

Unfortunately there are a number of posts that were published on the wrong day, so instead of updating them (and adding an alias to prevent permalink breakage) we'll just leave them as-is.

Closes #531 (closed).