Project-level integration: Implement project-level setting selector
As part of the iteration 1 #213857, we need to adjust the behavior of the project-level integration page.
We have to implement project-level frontend selector and backend behavior:
Use instance level settings, set
inherit_from_id = instance.id
Use custom settings, set
inherit_from_id = nil
In any case, we have to pre-populate the values with the instance-level values.