Bug 9988: Database revision for AuthorityMergeLimit

Bug 9988: Database revision for AuthorityMergeLimit

Introduce the new preference and remove dontmerge.
If dontmerge was enabled, the new pref will be set to zero and we will
print a message about raising this value now.

Test plan:
Run the database revision or a new install.
In case of an upgrade, check the new pref value. Should be 0 when
dontmerge was enabled, otherwise 50.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Also tested sysprefs.sql on a new database.

Signed-off-by: Marc Véron <veron@veron.ch>
Signed-off-by: Jacek Ablewicz <abl@biblos.pk.edu.pl>
Signed-off-by: Julian Maurice <julian.maurice@biblibre.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>