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 on the same machine. Since there were no permissions, repository management was admin-only, and you couldn't mirror, this was fine.

As we've evolved, this use case is a tiny minority use case and the security implications of `file://` URIs overwhelm the utility it provides. Prevent the use of `file://` URIs. Existing configured repositories won't stop working, you just can't add any new ones.

Also prevent `localPath` from being set via Conduit (see T4039).

Test Plan:
  - Tried to create a `file://` repository.
  - Tried to create a `file://` mirror.
  - Tried to create a `file://` repository via Conduit.
  - Created a non-`file://` repository.
  - Created a non-`file://` mirror.
  - Created a non-`file://` repository via Conduit.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D9513