USE INDEX(wb_epp_entity) in EntityPerPageTable::listEntities

MariaDB tends to use the `epp_redirect_target` key which has a very
low selectivity.

Change-Id: I0d9c6cde3f23bcd52eaad7ad9e0b0fc3aad3b9e0
Merge "USE INDEX(wb_epp_entity) in EntityPerPageTable::listEntities" into wmf/1.25wmf11