Bug 13766: Change *bibioitems.ean to mediumtext and add indexes

Bug 13766: Change *bibioitems.ean to mediumtext and add indexes

- biblioitems.ean and deleteditems.ean are changed
  to mediumtext to match issn and isbn.
- An index is added for ean on both tables.

Patch cleans up inconsistencies caused by bug 5337 that
causes some older databases to have an existing index
on biblioitems.ean.

Signed-off-by: Colin Campbell <colin.campbell@ptfs-europe.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>