Add Visual Review instructions to the MR Widget

What does this MR do?

This MR covers adding the app portions for the Visual Review App MVC (see #10761 (closed) and &960 (closed)). It adds a button to the Deployment footer in the pipeline section of the MR widget, which triggers a modal containing the relevant information.

Still To Do

• Internationalization pass for the text (I assume I need to add the strings somewhere and wrap all of them.)
• Placing behind a feature flag
• Figuring out what I need to do to set it as available to a certain level
• Tests
• Revert forcing commit

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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