cmd/upspinserver: serve only 'all' content through web interface
cmd/upspinserver: serve only 'all' content through web interface

Fixes #192

Change-Id: I6bf4761984170d0c45322e6fff28c5407a1e5c9d
Reviewed-on: https://upspin-review.googlesource.com/8304
Reviewed-by: Andrew Gerrand <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

upspin/upspin,upspin/upspin