Readonly enum values

This is to address:

  https://www.pivotaltracker.com/story/show/49272069

Which is, paraphrased: There's a hard-coded list of event types in the accessions controller that get used to display the list of common event types when creating an accession.  Since it's hard-coded, things go pear-shaped if the user deletes those enumeration values.

This change allows individual enumeration values to be marked as read-only.  The backend will throw an error if you try to remove or migrate the value, and the frontend won't show the migrate or delete buttons for those values.  The special values are still listed in the accessions controller, but I've switched them to be read-only.