CORE-5386 Fix for TreeSelection value conversions.

Added conversion for arguments in the tree's root.
CORE-5832 Ensure pipeline steps returned in order.
CORE-5832 Initial /app/:id/ui endpoint conversion.

The /app/:id/ui endpoint now returns JSON for single-step apps, and edit/copy pipeline endpoints are now separate endpoints.
Added parameter_values and fixed validation_rules for the parameters entity in kameleon.