Allow Mixtape project owners to add and remove people from their project channels

[jacksonargo]

My initial idea is to use slash commands to do this. Something like

/mixtape my_mixtape_channel add|remove @some-interested-user

[jacksonargo]

I'm going to scratch the idea of slash command rn now, and probably focus on web ui. ReactJS has made it much easier to implement things form ui perspective, so I actually like the workflow of make a web ui first , then add slash command as feature++