R1:c7550dbee9b1

Fix Phriction document previews for the root document ("/") with Apache option "MergeSlashes On"

Summary:
Ref T13662. I ran into this while trying to reproduce the mention issue discussed there.

Currently, the root document (with slug "/") attempts to preview using the URI `/phriction/preview//` (with two `//` at the end). This is collapsed into "/phriction/preview/" by Apache if "MergeSlashes On" is configured, which is the default behavior. The route then 404s.

Instead, just use "/phriction/preview/?