Honor custom notification settings when participating on an issue

What does this MR do?

Relates to #271252

If a user has custom notifications set at the project, group or global level, some settings will be ignored given that the user is participating in the issue that triggers the notification.

We should always honor the user's notification settings, even if we become participants of an issue. Mentions are the exception as they can't be disabled as a notification setting.

Screenshots (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
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Security

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Related to #271252