Bug 9722: Allow users to add notes when placing a hold in opac (dbrev)

Bug 9722: Allow users to add notes when placing a hold in opac (dbrev)

Database revision for report 9722, adding new pref OpacShowHoldNotes.
This development is part of a larger one (see umbrella report 9721).

Test plan:
Run [your server]/cgi-bin/koha/installer/install.pl?step=3&op=updatestructure
Optionally, run a new install to test sysprefs.sql.
Or:
Edit sysprefs.sql, leave at least the last few lines including the new one.
And run from command line:  mysql -p [yourdatabase] < [your-clone]/installer/data/mysql/sysprefs.sql

Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Jonathan Druart <jonathan.druart@biblibre.com>