Display "folders" for environments

Description

With 8.12, we'll start to see a proliferation of environments that really should be grouped. This is particularly important for review apps.

Proposal

• Environments that are named like type/foo should be presented under a group named type.
• For now, don't interpret anything special based on the type; just sort alphabetically.

Single Source of Truth

spawning review app environments should be in order of updated

Links / references

• Master issue: #20255 (closed)
• Following on from: https://gitlab.com/gitlab-org/gitlab-ce/issues/21971#convention-over-configuration

/cc @ayufan @dimitrieh