Make it more explicit what environment stop actions are

What does this MR do?

This merge requests attempts to resolve a technical debt related to environments / deployments and stop actions. It makes it explicit that environment is not a stop action, instead it merely contains a stop stop that is sometimes available depending on the environment state.

Similar case for deployments.

Why was this MR needed?

We worked with @winh and this was not clear what is a stop action and how it relates to environments.

Checklist

• Tests added for this change
• Has been reviewed by a Backend maintainer
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#48672