Disable Account Registration Config

[jacobtread]

Description

For larger servers being able to register an account through the dashboard might not be something that is ideal as users may spam create accounts to fill up the database or reserve names

Proposed Solution

Adding a disable account registration configuration setting to the server configuration.

Checklist

• [x] Account registration can be disabled
• [x] Server create account endpoint responds with an error when registration is disabled
• [x] Disabled account registration is visible on the dashboard

[jacobtread]

This issue has been altered and will no longer depend on #36