Only email users about alerts when status is triggered or resolving

What does this MR do?

Related issues: #299961 (closed), #262866 (comment 496628300)

Changes in this MR:

• Only email users about incoming alerts when the alert is triggered
  • There are 2 notification features affected: Email notifications and On-call management
• Removes logic which re-triggers from prometheus alert processing

With this MR, the documentation for Email notifications is now accurate, and it wasn't technically before.

The on-call management pieces are still behind a feature flag, so they're not mentioned in the changelog here & that's why the changelog is not only in EE.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• Separation of EE specific content

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
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team