Enable editing of post date

[keybits]

It can be useful to edit the date of a post. Currently it's fixed in the UI to the date you first save a post.

Reasons it can be useful include:

• adding old posts when migrating from another site
• publishing a post that was initially created as Private (a draft), it makes more sense to show the date the post was first made public

Originally posted by @keybits in https://github.com/PostOwl/postowl/discussions/51