Backwards compatibility is strictly preserved in `__init__`, including parameter names, defaults and position. Whenever that's difficult using `attrs`, the custom `__init__` is kept instead (for the time being).
`__repr__` methods have changed to the format `attrs` use, but people don't rely on this for anything other than debug output, so it shouldn't be a problem.
* Move exception models into separate file
* Move thread model into separate file
* Move user model into separate file
* Move group and room models into separate file
* Move page model into separate file
* Move message model into separate file
* Move basic attachment models into separate file
* Move sticker model into separate file
* Move location models into separate file
* Move file attachment models into separate file
* Move mention model to reside with message model
* Move quick reply models into separate file
* Move poll models into separate file
* Move plan model into separate file
* Move active status model to reside with user model
* Move core enum model into separate file
* Move thread-related enums to reside with thread model
* Move typingstatus model to reside with user model
* Move emojisize and reaction enums to reside wtih message model
