Damage from a given individual or group only counts towards a kill for 10 minutes since that individual or group's last damage to the entity - after this period their damage contribution is removed. This avoids the list of damage contributors growing excessively large for an entity that does a lot of combat but never dies.
EXP sharing within groups is unchanged - the difference is simply that the input to this calculation may be less than 100% of the base EXP reward for the kill if other individuals or groups contributed damage.
- Separate `invite` machinery from `group_manip` into it's own thing (includes renaming `group_invite` to `invite` where applicable).
- Move some invite/trade machinery to `ControlEvent`.
- Make `TradePhase` a proper enum instead of a bunch of bools.
- Make `TradeId` a proper newtype.
- Remove trades from `Trades` on accept (previously was only on decline).
- Typo fixes/misc cleanup.
- Add bullet point for trading to the changelog.
