Bug 8744: Thesaurus in authorities should not lock fields

Bug 8744: Thesaurus in authorities should not lock fields

Although fields in the authority editor should support thesaurus
control, they should never be locked, as authority records commonly
refer to other records that may not be in use locally (and therefore
might not be in the authority file).

Test plan:
1) Make sure a 5xx field in one of your authority frameworks is
   thesaurus-controlled.
2) Note that you cannot edit the field directly.
3) Apply patch.
4) Note that you can now edit the field irectly.

Signed-off-by: wajasu <matted-34813@mypacks.net>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>