Added flag `manualHeroVisit` flag to town building. If this flag is set,
then building will only be activated on click and will not give its
effect on hero recrutiment, hero visit, or new day.
This allows implementing changes to Mana Vortex from HotA
- Fixed off-by-one error when checking for obligatory skills
- If both wisdom and magic school must be offered in the same slot, magic
school will be correctly offered on next levelup
- Obligatory skill can now be proposed for upgrade
- Obligatory skills are now offered using hero class weight instead of
simple random
- If hero has multiple skills not available to his class game will
select random skill instead of first one
- Moved storage of random seed to server instead of mutable member
