Alert list loading & error states

What does this MR do?

Adds loading and error states to for alerts

🇺🇦 Alerts section is behind feature flag: make sure to run Feature.enable(:alert_management_minimal) in rails console

When enabled, navigate to "Your project" => Operations => Alerts

⚠ Currently graphql endpoint is not merged, so for this MR it will display loading state, then error state

Successful list loading will be implemented in scope of this issue - #213880 (closed) after BE MR is merged

Screenshots

Loading

Error

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

Related to #213890 (closed)