R1:87ff461470de

Add /F123 shortcut

Summary:
I wanted to point someone on a file uploaded to Phabricator and the normal link is just too long.

I guess that this also improves security. Because pointing someone to the file directly reveals the secret key used in /data/ and it can be served without auth?

We already use `{F123}` so there will be no conflicts in future because we wouldn't want to reuse it for something else.

I promote the link on /file/ - it adds one redirect but I think it's worth it. I also considered…