Improve the 'review' job and make it manual

What does this MR do?

1. In order to investigate https://gitlab.com/gitlab-org/gitlab-ee/issues/6884 properly, this change the review job to be manual.
2. This also merges the review-app-image job into the review job because there's no real reason to put review in the post-test stage. => I abandoned this idea for now since the charts Docker image doesn't seem to have a recent Ruby installed (this can be improved in a later iteration).
3. Lastly, this extract the deployment shell functions to a separate file to improve maintenance, and it also makes the file CE-ready by default the Rails images to the CE ones, and using the EE images in the case of an EE pipeline.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Because Review Apps aren't working as expected right now:

• omnibus-gitlab#3683 (closed)
• https://gitlab.com/gitlab-org/gitlab-ee/issues/6884

Does this MR meet the acceptance criteria?

• Conform to the code review guidelines
  • Has been reviewed by a Backend maintainer
• EE specific content should be in the top level /ee folder
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?