Limit alert assignment to only users who can read alerts

What does this MR do?

This MR adds error handling for alerts being assigned to users who cannot read the alert. The UI will then show an error if the user attempts to do so.

Screenshots

In this video, the only user on the project is root - all others should throw and error. Combining this with !34649 (merged), the only users who show up in the list but will cause an error on assignment are guests and reporters.

Display_error_with_bad_assignment

Related issue: #222672 (closed)

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