Apply carryover before displaying mp::connect between scenarios.
Allow "Custom" faction for empty recruit lists.

This allows to properly display faction combo and makes a non-hacky way
to specify a side without recruit list.
Add a lock attribute for [multiplayer] sides.

The new "side_configurations_lock" attribute is used as a default value for
gold, income, team and color locks. Later, it should be used to lock
factions and leaders combos as well.

The attribute's default values are "no" for the first scenario and "yes"
for all the other ones.