Improved email whitelisting for self-hosters

[Kritik-J]

Closes #833

✅ Checklist

• I have followed every step in the contributing guide
• The PR title follows the convention.
• I ran and tested the code works

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Changelog

• Reject GitHub OAuth users not on the email whitelist
• Implement email whitelist check before sending magic link emails

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Video

email-whitelisting.mp4

💯

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Latest commit: 182adc5

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