Re-implement Review Apps 2.0 (with correct environments)

We were very close with !3179 (merged), but turns out we forgot one variable in the environments so it wasn't working. This brings back the original changes, and fixes the environment issue.

This should be merged around the same time as the following, to make sure the review apps from those 4 other projects work:

• Set variable to new domain for docs review apps... (gitlab!101594 - merged)
• Set variable to new domain for docs review apps... (gitlab-runner!3671 - merged)
• Set variable to new domain for docs review apps... (omnibus-gitlab!6465 - merged)
• Set variable to new domain for docs review apps... (gitlab-org/charts/gitlab!2819 - merged)

Related Issues/MRs

• Related to #735 (closed)
• Related to !3179 (merged)