Pages need a "created_at"/"first_published_at" for rapid query/ordering purposes

Similar to #577. Sometimes you need to filter pages of a type and order them by descending date, as a crude way to feature the most fresh content first.

When the date of creation is something you want the public to know/see e.g a blog posts or a press releases, dates warrant a field of their own in your custom model, but most of the time you just need an approximate measurement of freshness to use procedurally in the background, and don’t want to have to add a field to store it.

I’d like a “created at”, and also a “first published at”, because the date of creation could with some workflows be far from the date of publication.

(The existing latest_revision isn’t useful here because you don’t want old pages appearing first after receiving a minor grammatical tweak.)

[nadersoliman]

first_published_at is implemented with editable=False and as such cannot be added as a UI field in any page editing tab, so why the decision of editable=False in first place?

[chrxr]

Merged in https://github.com/torchbox/wagtail/pull/1054. Opened new ticket for UI implementation discussion https://github.com/torchbox/wagtail/issues/1126.