Show Alerts list when Alert Service is enabled

What does this MR do?

Adds logic to show/hide Alert Management splash page based on whether the Alert Integration setting is enabled.

To exercise this code:

• Enable the feature flag alert_management_minimal
• Enable/disable the Alerts Service integration at <project-slug>/-/services/alerts/edit. (Try both!)
• See the results by loading the alert management page at <project-slug>/-/alert_management/.

Closes #215573 (closed)

Original discussion: &2877 (comment 337107590)

Note: This feature is behind the alert_management_minimal feature flag.

The changelog will be added when feature flag is removed (MR here !31171 (merged)).

Docs are added in this MR !31225 (merged).

Screenshots

Alert Service disabled	Alert Service enabled

Does this MR meet the acceptance criteria?

Conformity

• [n/a] Changelog entry
• [n/a] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [n/a] Database guides
• [n/a] 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
• [n/a] 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:

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