Avoid opening 2 modals for enabling review app

What does this MR do?

The bug is described in #263402 (closed) (video available in Before column)

This MR corrects a misleading usage and naming of a component which caused the modal to be triggered twice:

• moves the GlButton out of the component with GlModal
• renames the component to clarify it contains a modal (only)
• adds / updates tests to ensure enable-review-app-modal is never rendered more than once

Screenshots

before	after
gitlab_issue2	Screen_Recording_2020-10-16_at_09.53.18

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] 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 #263402 (closed)