Create the delete modal for 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 creates the delete modal for status checks:

• Added new delete action to communicate with the REST API
• Added new click event to the remove button
• Added new modal to the status checks app which is opened by the delete event
• Created a new delete modal to render the warning text and trigger the delete action on confirmation

Screenshots (strongly suggested)

Grabación_de_pantalla_2021-05-14_a_las_11.06.29

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Setup & testing

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. Add status checks if none exist
4. Click the "Remove..." button and check the modal appears and can be cancelled/submitted
5. Check the status check is no longer on the status checks list
6. Cycle through clicking different status checks "Remove..." buttons and confirm the modal details change correctly

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)