Allow users/mods to add expire date to a post

[Saulios]

Allow users (probably the poster and mods) to make a specific comment to add an end date to a post. Adds it both to post flair and flairs post as expired after that date.

Potentially it will be possible on time, but that's difficult with timezones. Requires a way to store data, so no plan to add this anytime soon.