[Feature Request] Make "stayinchannel" a Guild Setting

### Is your feature request related to a problem? Please describe.

I'm hosting MusicBot for a couple friends' guilds. Some want the bot to stay after its queue ends, and some want it to leave after that.

### What is your ideal solution to the problem?

Add a `stay_in_channel` setting to guilds, similar to `prefix` or `skip_ratio` etc.

### How would this feature be used?

1. Server admin would run a command to chance stayinchannel setting.
2. Bot would respect that setting in that guild.

### Additional Info

#970 is somewhat related. Though I'm not asking for a new setting, or to break compatibility for an existing setting. Just a way to set a setting per-guild.

### Checklist

- [X] I have checked [the documentation](https://jmusicbot.com/) to make sure this feature doesn't already exist
- [X] I have searched for [similar feature requests](https://github.com/jagrosh/MusicBot/labels/enhancement)
- [X] I have confirmed that my requests is **not** on the [list of things that will not be added](https://jmusicbot.com/things-that-wont-be-added)
- [X] I am running the latest version of the bot: 

Start by locating how existing guild settings such as prefix and skip_ratio are stored, changed by admin commands, and read when the queue ends. The feature is done when an admin can change stay_in_channel per guild and the bot stays or leaves according to that setting; review #970 for related behavior.

Written by the indexing model from the issue text.