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.
7 jobs for feature/publish-dates in 4 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #235194924
build_deploy_image

00:01:16

passed #235194922
build_site

00:01:33

passed #235194923
build_test_image

00:00:54

 
  Review
passed #235194925
review_deploy

00:00:56

passed #235194926
allowed to fail manual
review_stop

00:00:33

 
  Test
passed #235194927
test_image

00:01:13

passed #235194928
allowed to fail
test_links

00:01:30