add custom voice channels

[danthe1st]

This PR adds custom voice channels to the bot.

When a user joins the template VC, they get a copy of the VC with MANAGE_MESSAGES permissions and are pinged with some information about custom VCs. Users can:

• edit VC settings via Discord (not permissions)
• toggle whether the VC is private or public (using a button)
• add/remove users to/from the VC (using /vc-control)
  • removing the user kicks/moves them to the AFK channel