Bug 5642: Item field serial enumeration (enumchron) should be longer

Bug 5642: Item field serial enumeration (enumchron) should be longer

Field enumchron in items table is now varchar(80).
We have records that need a much longer field, even up to 400 or 500 chars.
I suggest to change its type to TEXT (variable length with max 64K; tinytext
goes up to 255 chars and is just too short).
Mediumtext or longtext are not needed; as a side note these types are used in
the items table for e.g. booksellerid and more_subfields_xml..

Revised original patch thanks to Ian Walls: update kohatructure.sql. Copied the change in deleteditems as well.

Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Signed-off-by: Chris Cormack <chrisc@catalyst.net.nz>