Make graphite's /render/?.... urls copy-pastable

Links to graphite images are completely unpastable to jabber/irc/mail as
clients mangle the humongous urls. These commits add a url shortener that can
shorten any graphite url from /what/ever/.... to /S/<base62_string>.

It can be used manually by prepending /s to the url path, and I added a button
to the graphite composer that does this for you.