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 making the link from the File ID column but there are already too many links (with some duplicity).

Test Plan:
/file/
/F123 (redirect)
/F9999999999 (404)

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin

Differential Revision: https://secure.phabricator.com/D2380