[Suggestion] Use vanity URL for partnered servers for public listing

[vladfrangu]

That way, we don't make unnecessary invites for partnered servers!

[Gilbert142]

Good idea, will push on v4.0.2 or v4.0.3

[vladfrangu]

Endpoint: GET /guild/id/vanity-url
Requires the bot to have manage guild, so rather, we'd make server admins choose between giving gab manage guild to check the vanity url OR make them manually enter it and then use client.fetchInvite to see where it goes, invalid inv / wrong guild => error