R1:8ad61d01502d

Simplify "builtin file" management and recover from races

Summary:
Fixes T11307. Fixes T8124. Currently, builtin files are tracked by using a special transform with an invalid source ID.

Just use a dedicated column instead. The transform thing is too clever/weird/hacky and exposes us to issues with the "file" and "transform" tables getting out of sync (possibly the issue in T11307?) and with race conditions.

Test Plan:
- Loaded profile "edit picture" page, saw builtins.
- Deleted all builtin files…