Implement the cancan filters for diary entries

Implement the cancan filters for diary entries

Access logic is not _entirely_ exported from the controller,
unfortunately.  For interface reasons, some actions which require admin
have to be listed within the controller's deny_access method.

This is required because, being a default-deny system, cancancan
_cannot_ tell you the reason you were denied access; and so
the "nice" feedback presenting next steps can't be gleaned from
the exception