issue #25 - made collection/entity id fields non-editable

The fields still need* to be written to JSON and EAD/METS so they
have to remain in fields lists, but the fields are now hidden.
IDs are still displayed in the interface.

* Actually this is debatable. As long as we use collection/entity
ID in the directory names then these would be guessable.  But of
course that may change.