Add ability to fully delete a post

[durden]

We currently just soft delete a post. This seems fine, but it brings up a few questions:

• Should we have a scheduled job to run through and delete 'deleted' posts over a certain amount of days/weeks old?
• Should we just remove the soft delete functionality and delete it completely?

This soft delete concept is not obvious in the interface or the API at all. The following steps demonstrate the confusion:

1. Create/save a post with a slug of 'my-title'.
2. Delete this post from the UI.
3. Create a new post with a slug of 'my-title'

At this point you'll get an error saying you already have a post with this slug. However, from the user's perspective this post doesn't exist anymore.