Move alert management behind a feature flag

What does this MR do?

Incomplete alert management splash page and sidebar entry was recently merged to master - !29775 (merged).

Existing functionality shows a splash page, but the underlying list view is empty and doesn't yet fetch alerts and list them. This means the currently merged state is not yet ready for customer use.

This MR moves the incomplete alert management functionality behind a feature flag: alert_management_minimal.

Also fixes a bug I discovered while I was adding this - if the alert management sidebar display conditions are not met, Tracing and Environments menu items were being hidden in the sidebar. This has been solved.

Closes #215417 (closed)

Related issue: #213880 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• 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