Empty state for alerts list

What does this MR do?

Creates empty list view for alerts

⚠ Currently there is no BE call, cause endpoint is not ready. It will be implemented in scope of the next issue

⚠ ⚠ GlTable styles are overwritten in this MR, cause they inherit from GitLab styles - check here with Include GitLab CSS bundle checked.

Screenshots

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

References #213892 (closed)