Bug 8480: Add constraint on auth_subfield_structure.authtypecode

Bug 8480: Add constraint on auth_subfield_structure.authtypecode

In order not to have useless entries in the auth_subfield_structure
table, this patch modifies the DB structure to add a foreign key on the
authtypecode column.
Note that the auth_tag_structure already has this constraint.

Test plan:
0/ Don't apply this patch
1/ Create a now authority type 'RM_ME'
2/ Look at the MARC structure, to create the subfield structure and
populate the auth_subfield_structure table.
3/ Delete the authority type
4/ Using your SQL cli:
 SELECT COUNT(*) FROM auth_subfield_structure WHERE authtypecode='RM_ME';
=> The data are still in this table.
5/ Apply this patch
6/ Execute the updatedb entry
7/ Confirm the entries in the auth_subfield_structure table related to
RM_ME have been deleted
8/ Repeat 1, 2 and 3 and verify the auth_subfield_structure entries have
been correctly removed.

Signed-off-by: Frederic Demians <f.demians@tamil.fr>
- Works exactly as described in the (very good) test plan.
- After authority type deletion, auth_subfield_structure still contains
  entries for deleted authority type. Applying the patch clean the
  previously undeleted records in auth_subfield_strucute. Now deleting a
  authority type cleans propertly all appropriate records in
  auth_subfield_structure.
- Fix a merge conflict

Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>