- 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.
