Add listing app to show status checks

What does this MR do?

With #327638 (closed) we are pivoting the status checks from being developed as part of the MR approval rules section, to its own dedicated section. This is because status checks don't "approve" an MR but rather give the MR author/merger an idea as to whether the MR passes any external processes. Putting it in a separate section helps keep the distinction that you don't need a status check to pass to merge an MR like you do approval rules.

This MR specifically setups the store, mounting HAML points and the core listing app to render status checks. Future MR's will add the add/edit/delete functionalities.

Screenshots (strongly suggested)

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Empty
With services

Setup & testing

Note: Right now, adding/editing/deleting status checks is functioning within the approval rules section as per previous work. Use this area to control the status checks for now until future MR's move the functionality to the new area

1. Enable the feature flag :ff_compliance_approval_gates
2. Go to [HOST]/gitlab-org/gitlab-test/edit and expand the Merge requests section
3. Check that Status checks is empty
4. Open the Merge request (MR) approvals section and add a new approval rule using the type Status checks
5. Check that Status checks contains the added status check

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
  • I have included a changelog entry.
  • I have not included a changelog entry because this is behind the :ff_compliance_approval_gates feature flag.
• [-] 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 #327638 (closed)