Allow "[stats]" unique on terrains  - By SeventhM

By SomeTroglodyte:
- PolicyPickerScreen description links to Civilopedia
- Fix music errors on android pause-via-homescreen-button
- Remove UniqueTarget.Terrain from UniqueType.Stats as there is no implementation

By PLynx01:
- Added new trigger unique "Remove [buildingFilter] [cityFilter]"
- "when above [amount] [stat]" conditional, with gamespeed-modified version