R1:ab20f243b341

Improve consistency of file access policies, particularly for LFS

Summary:
Ref T7789. Currently, we use different viewers if you have `security.alternate-file-domain` configured vs if you do not.

This is largely residual from the days of one-time-tokens, and can cause messy configuration-dependent bugs like the one in T7789#172057.

Instead, always use the omnipotent viewer. Knowledge of the secret key alone is sufficient to access a file.

Test Plan:
- Disabled `security.alternate-file-domain`.