!vc: Add ephemeral text channels to go with the ephemeral voice channels

[mythmon]

When a user creates a voice channel with !vc create Let's play a game, that should also create a paired text channel, #lets-play-a-game-chat. When the voice channel is deleted, the text channel should also be deleted.

Only users that are currently in the voice channel should have access to see the chat channel, which is primarily intended to avoid spamming users that have notifications default to on for new channels.

The text channel should have a description that makes it clear the contents are ephemeral, and won't last longer than when the channel is cleaned up.