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
There is no need in converting the result from `OC_Util::generateRandomBytes` since it returns a printable string. Additionally, https://github.com/owncloud/core/pull/10642 will deprecate this call.