Change default of `restrict_user_defined_variables` to `true` behind FF

Refactor namespace variables default role migration to enum
Adjust default role reference from namespace to namespace_settings
Validate default pipeline variabe role on namespace settings
Restore nil allownace for the role on namespace
Move FF to namespace_settings and use no_one_allowed as default role
Fix typo in index.md
Adjust index.md docs
Restore qa tests
Leave only enum validation on namespace settings
Define DEVELOPER_ROLE in the transaction
Replace after_initialize with before_create
Move Chain Associations to 4th line
Remove restrict_user_defined_variables update to true in tests
Fallback to default if no namespace settings defined, address tests accordingly

Changelog: changed