[Self-hosted] Single community mode guild auto-join not functional when approval is required

Summary

If approval is required and the user does not use a registration URL to create an account, the user will not auto-join the single community Fluxer instance's guild. I was able to auto-join with both open registration and closed/approval-required + a registration URL, but not approval-required + no registration URL. Confirmed this behavior on multiple instances.

@zixhaa I confirmed the same test with the same parameters. When set to "Closed", auto-join worked when using the Admin-Issued Registration URL with "Approval required" unchecked , it auto-joined the guild upon completing registration. When testing it with "Approval required" checked , then it created the account but did not auto-join the guild and left the account hanging without a guild.

Confirmation also posted by @whenbearsreign in #583 Issue was possibly closed incorrectly in #683; issue does not look like a duplicate of #682. It is a separate issue as it is a bug regarding the auto-join of the single community guild once account is already created (not using a guild invite link to create an account), not a bug in account creation (account creation proceeds without issue).

Steps to reproduce

1. Set instance to single community
2. Set instance to approval required
3. Create a new account from the base registration page and approve it