Skeleton route and view for alert management details page

What does this MR do?

⚠ NOTE: This functionality is behind the feature flag alert_management_minimal. Enable this to test functionality locally. ⚠

This MR:

• Adds a skeleton view for the Alert Management detail page.
• Adds a new route: alert_management/<id>/details
• Adds specs for the controller that processes this route.

This will allow numerous development tasks to continue in parallel:

• #213911 (closed)
• #214550 (closed)
• !30565 (merged)

Related issue: #213911 (closed)

Screenshots

before	after

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