Bug 31846: Add syspref for serials search results limit

Bug 31846: Add syspref for serials search results limit

To test:

1) Make sure SerialsSearchResultsLimit syspref is unset or set to 0.
2) Perform an advanced search on serials without any conditions
   and confirm all serials are listed as expected.
3) Set SerialsSearchResultsLimit to a value less the the number
   of total subscriptions, perform the search again, and confirm
   that the number of serials has been limited to the set value.
4) Ensure all tests pass in t/db_dependent/Serials.t

Sponsored-by: Gothenburg University Library
Signed-off-by: Kelly McElligott <kelly@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>