Bug 11165: Automatically save the authority last modification time

Bug 11165: Automatically save the authority last modification time

This patch rename the column auth_header.datemodified with modification_time.
Now this column will be automatically updated when a row will be
updated.

Test plan:
0/ Verify that la major part of your auth_header.datemodified is set to
null
1/ Execute the updatedb entry
2/ Verify that the column is renamed and the values have not been
changed.
3/ Update an authority and verify (using the sql cli) that this value
has been updated.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
In the absence of a coding guideline stipulating a preferred column
name for a timestamp, this modification_time works for me.
Null values are replaced by current time after db rev; other values
are left alone.
NOTE FOR RM: Please run update_dbix_class_files.pl.

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@gmail.com>