Allow managing Voice Activities

[Luc1412]

The Problem:

Currently you can't really know whether users using an activity in a voice channel. The only way is checking the user's presence. This only works if the user is online and enabled showing their presence.

Also some servers might restrict some activities like YouTube or Poker if the server is focused on a younger community.

Possible API improvements:

• Provide active activities in the channel object
• Add a new event or provide activity changes to an existing endpoint
• Add a way to moderate activities eg. stopping them. Kicking all members might be also fine.