Discord: add apiBaseUrl

[TheArcaneBrony]

I have no idea if I did this right, or whether I should be introducing a new badge type here.
Just quickly whipped this up in the web editor, as I wanted to have support for discord-compatible alternatives like Spacebar or Oldcord.

Example:
https://api.old.server.spacebar.chat/api/guilds/1006649183970562092/widget.json

[github-actions]

	Warnings
⚠️	This PR modified service code for discord but not its test code. That's okay so long as it's refactoring existing code.
⚠️	This PR modified service code. Please run tests by including affected services in the pull request title.
⚠️	📚 Remember to ensure any changes to config.private in services/discord/discord.service.js are reflected in the server secrets documentation.

	Messages
📖	✨ Thanks for your contribution to Shields, @TheArcaneBrony!

Generated by 🚫 dangerJS against 3bd1c8e

[TheArcaneBrony]

Not sure it makes sense to update the tests for this, given it doesn't really make sense to test against a selfhosted platform that doesn't have a "main instance"?

[PyvesB]

Just quickly whipped this up in the web editor

A fair bit more work would be needed to get this to work. 😉

In particular, you'll need to add a query parameter to the badge so that users can specify the Discord-compatible host they're targeting, as well as an authorizedOrigins configuration option to prevent the main Discord API token to accidentally be sent to another endpoint. The Drone service can for example be used for inspiration.