Default New Pastes to Active
The form when creating a new paste wasn't ignoring the active flag. This
changes that so they use the default of True.
Don't Overwrite Paste URL on Save
Save was blindly overwriting the URL for the paste. This wasn't a problem
before because it was only being called on creation. When I updated some
pastes, however, the URL changed.
Implement Expiration Date
The paste model was modified by removing the expiration second count and
replacing it with an expiration datetime. The query for recent pastes
respects active and expired pastes.

This isn't a complete implementation. Expried pastes are still available
via their URLs.
Make Paste Slugs Case Sensitive
The slug in the model is case sensitive but the URL wasn't. This change
makes the URL pattern case sensitive.
Don't Show Expired or Deactivated Pastes
Any paste that isn't marked as active or has expired will 404 on its URL.
Make Expiration Date Blankable
The admin page enforced the blank=false rule on this field. That wasn't
intended.