Add optional link to signup page when signups are closed

[sirilyan]

Add a config option that lets you set a URL to link people to when they click Sign Up on a server that's invite-only. Can point to @johnHackworth's Google Form, @sirilyan's kobold-page status page, etc.

Wording something like this:

"Read this page to learn more about invitations."

• [ ] option is added
• [ ] URL shows up on invite code window
• [ ] invite code window continues to work with no URL set
• [ ] i18n: English
• [ ] i18n: Spanish

[sirilyan]

It is possible this could be completely done in the locale files.