Add feature specs for multiple environment feature flag

What does this MR do?

• Add feature specs for multiple environments support for feature flags
• Improve validation in case frontend tries to insert a default scope to a second, third row
• Sets the default value for feature_flag.active in case frontend doesn't send active value. This is necessary because frontend doesn't send active value in the new behavior, whereas they sends it in the old behavior
• Do not create a default scope if frontend has already contained the information in the parameter scopes_variables. This is necessary to avoid a conflict between old and new behaviors.

This feature is behind a feature flag named feature_flags_environment_scope

What are the relevant issue numbers?

Related: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9228

Does this MR meet the acceptance criteria?

• [-] Changelog entry added, if necessary
• [-] Documentation created/updated via this MR
• [-] Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• [-] Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• [-] Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• EE specific content should be in the top level /ee folder
• [-] For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?
• Security reports checked/validated by reviewer