Make the AI recruit troops

[SamTheBlow]

Currently, the AI does not use troop recruitment at all. In some rulesets, it's viable, or even essential to recruit troops.

• [ ] If reinforcements are enabled and proportional to pop size, never recruit
• [ ] Recruit the maximum by default
• [ ] Recruit starting in the most populated (TestAI1) or most endangered (TestAI2) province
• [ ] If population growth is enabled, switch to smart recruit
• [ ] If income is proportional to pop size, switch to smart recruit
• [ ] Smart recruit: recruit just enough for the population to not go down
• [ ] Smart recruit: max recruit in case of emergency