Automatically publish on publication date

[beroset]

Our news items each have a posted field which has the date that the news item is intended to be posted. However, the news item is posted as soon as the pull request introducing it is merged. It would be convenient to be able to merge the item and then have the news item automatically become visible on the date.

I can see a few ways we could potentially address this:

1. use javascript on the browser to hide items until the date
2. set the date at build time based on the git commit date
3. change the meaning of the date to be "created" or "submitted" date rather than published date

I would be most inclined to use option 2, but there may be other ways to resolve this that are not enumerated here.