[iOS and Mac] Add handoff support

On MacOS and iOS there is a feature called [Handoff](https://support.apple.com/en-us/HT209455) that allows you to run a software on your computer and pick up right where you left off on your phone (or vice versa). I am proposing that Discord adds handoff support so you can be in one channel on your computer and then open that channel on your phone with your message transferred to the text field on your phone. This would make it easier and more seamless to walk away from your computer while not losing your place in what you’re writing.