Include minimal republication history

[deivid-rodriguez]

Currently when ads with comments are republished we get confusing dates. See the attached image, ad was published 12 days ago, but has a comment 3 months old.

To workaround this, I opened #513, which changes republications to delete previously existing comments in the ad.

But a better solution proposed by @andreslucena would be to add a new db field (original_published_at) and show that in the UI. Something like "Republished 1 day ago, originally published 3 months ago."