Shorter sharing links

:zap: WIP :zap: 

Opinions on this? This is (or should be) backwards compatible to 32 character hex tokens.

In combination with https://github.com/owncloud/documentation/pull/462 (and whatever apache has to do for that) we can get pretty neat links.

E.g.
https://yourcloud.com/s/3pyaiaav7bt2i
instead of
https://yourcloud.com/public.php?service=files&t=e91d871628e40fa23ac101a8e26805be
@LukasReschke Should we use medium for this?
I copied this from https://github.com/owncloud/core/pull/10755#discussion_r16935011 and realized just now, that I forgot to change it to medium.