Bug 8835: New db rev for issn_idx

Bug 8835: New db rev for issn_idx

This db rev reapplies dbrev 3.15.00.049 in case that one failed due to
existence of issn_idx.

Test plan:
If you do not have index issn_idx, recreate it:
    ALTER TABLE biblioitems ADD INDEX issn_idx (issn (255))
Run this db rev. You should see: Removed issn_idx.
Rerun the dbrev. You should see: Everything is fine.

Signed-off-by: Nicolas Legrand <nicolas.legrand@bulac.fr>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>