Remove irrelevant counteroffer notifications when the trade request is invalid

By EmperorPinguin: 
- AI: better explore logic 
- AI: reduce food weight for small cities 
- AI: able to choose instant heal promotions 
- AI: don't annex cities if it causes severe unhappiness 

Add 'on [difficulty] or higher' conditional - By RobLoach

By metablaster:
- Show religion display name when enhancement is made 
- Fix embassy trade offer crashes