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 UI and got a helpful error message.

Reviewers: chad

Reviewed By: chad

Subscribers: eadler

Maniphest Tasks: T10923

Differential Revision: https://secure.phabricator.com/D15899