Add Black Death disaster.

(cherry picked from commit dce65e4adcabd0c236bd365c4a0490f6067dc57b)
Remove unnessary whitespaces.

(cherry picked from commit c902711c6d5b4e9bbec24528f0507beed9e21554)
Add missing import.

(cherry picked from commit bebba5a0ab3aee28ad102b9c0600bec5e33d6c2b)
Deduplicate

(cherry picked from commit 72b9dfa95bdfc25432b761efd20a745f0a5b54a5)
Black death disaster kills/remove inhbaitants

from the related housing instead of destroying the building.

(cherry picked from commit 569e28777b8a1314cdea086e9d434b7a996b2fd7)
Move similiar disasters into one db table.

(cherry picked from commit c41e12c66279e06137c0903089b097b48c184dad)
Fix typo.

(cherry picked from commit d46a1422572b7bdcc77ac430fdcf54fb8448886a)
Create an own class to handle disaster influencing

the residents of a settlement.

(cherry picked from commit 335e3005c54d4fd65215dd302de2e60254ecf869)
Inhert disasters from BuildingInfluencingDisaster

(cherry picked from commit f2bfa39afe5167e9d169fed11309e6d6b9663213)
Move rescue building type to disaster classes.

(cherry picked from commit 5673570840ddda8fdece68e1deeed9f633079b71)
Fix multiple minor problems.

(cherry picked from commit 2aa381ec1b2d695f680ef26731cab597d4f6574a)
Add nonfunctional test case for the black death.

(cherry picked from commit 973d866576bffb077e01175785beca5a5f51b587)
Fix blackdeath test

(cherry picked from commit 5c80970056119e8f05819d326cfb885b4fdbd06e)
Fix bug in blackdeathdisaster and make trader ignore infected settlements

(cherry picked from commit d08bba6b75c990d06d30a9841832e7c2674a0285)
Make the black death disaster do something (fix test)

Made the black death infect buildings and let them recover after their
population has been reduced to 1.
The tset fixture now actually contains buildings worth infecting for the
test.

(cherry picked from commit c7514742eff8d72b3f0b3a8ea5063621679415c2)
Make the AI aware of the doctor building.

aiplayer.building.doctor needs more love.
Everything else needs balancing and testing.

(cherry picked from commit f2c5f05690fcd5ad010dbd9f3405f6daef9e5a6c)
Fix comment.
Make the AI build doctors.

Currently handled pretty much exactly likethe firestation
Hopefully make the AI build/buy herbs
Fix crash in cornercase when denying upgrades to a settler

The game would crash when denying upgrade permissions to a settler
which had previously started leveling up, but had then leveled down.
(Technically: had a upgrade production line active, but then leveled
down). When leveling down the upgrade production line was not removed.