R1:9d196648f54f

Prevent users from disabling repository builtin URIs

Summary:
Ref T10923. Currently, users can disable or enable builtin URIs, but this doesn't actually do anything.

The behavior of "disable" has changed a bit over time and might need some further refinement, but it's currently meaningless for builtin URIs. Prevent adjustment of it. If users want to hide a URI, they should set "Display: Hidden" instead.

Test Plan:
- Disabled/enabled a non-builtin URI.
- Tried to disable a builtin URI, saw greyed out…