feat: Notification rework

[Talanor]

Is there an existing feature or issue for this?

• I have searched the existing issues

Expected feature

Notifications :

• Should be user-based (vs site-wide), everyone should be able to configure their notification settings
• Only admins should be able to update api tokens however
• Notification vectors should include mail sending
• "Vulnerabilities Found" should be filterable by severity

Alternative solutions

No response

Anything else?

No response

[github-actions]

👋 Hi @Talanor,
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