R1:d401036bd82b

Prevent the use of `file://` URIs in Diffusion

Summary:
Via HackerOne. There are two attacks here:

- Configuring mirroring to a `file://` URI to place files on disk or overwrite another repository. This is not particularly severe.
- Configuring cloning from a `file://` URI to read repositories you should not have access to. This is more severe.

Historically, repository creation and editing explicitly supported `file://` URIs to deal with use cases where you had something else managing repositories…