Right now this is just an attribute on the dataset. I considered making
it a distinct dataset type, but I think that is overkill and would require
additional work (we don't support changing the type of a dataset).

Things to know:

 * You can only immortalize a 'valid' dataset. If it is already in 'grace'
   or 'locked' you will have to get it to the valid state first. This might
   be a problem if we want people to be able to create a dataset that is
   permanent right off the bat. I do not favor that.

 * Only admins can change the permanent status of a dataset.

 * This only handles the backend ("Classic") interface. Leigh will plumb
   it through the portal.

I also took the sky-is-falling edge off of the email we send to users
when a dataset enters the grace period. At least for those that we really
are not going to destroy.

And I clear my internal debugging "why" attribute whenever a lease is
put back into the 'valid' state from whereever it was.