Move Dropdown to Stick to MR View App Button

What does this MR do?

The dropdown of changed pages in the review app should be attached to the view app button. This moves it to the correct place. The Review App button is to the right of both of those.

closes gitlab-org/gitlab-ee#12184

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content

Performance and testing

If cross-browser testing is not required, please remove the relevant item, or mark it as not needed: [-] -->

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers