Support runtime Visual Review configuration

Problem to Solve

The Visual Review apps feature is nice but very difficult to use because you have to hardcode runtime configuration into your branch, which will result in merge conflicts and having to remember to remove the config before it merges, amongst other strangeness.

Proposal

For the next iteration we should prompt the user for the MR number they want to report on if no value is available at runtime. For teams who do have a working flow where they want to hardcode a merge request number per branch they can continue to do so, but if it is not provided it simply prompts.

Design

• Merge request ID input step
  • Enter your merge request ID
• Reuses existing UI
• Link links to documentation explaining why we need the merge request ID.
• Added in text in last state: Reports to merge request [!12343](URL). You can change that [here](URL)
  • !12343 links to the merge request
  • here links back to merge request id configuration state

Note: Modal UI changes have been moved to gitlab-ee#12447

Reference

• As a followup to this discussion: https://gitlab.com/gitlab-org/gitlab-ee/issues/10761#note_164273318