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
> seems like what I need. I'll implement that as soon as #10642 is merged and will remove my base_convert stuff afterwards.

I'll expose a few default charsets (e.g. alphanumeric, etc...)

> This means I'll need to do some recalculating on the math and $tokenLength levels though

Mhm, why we just don't define the token length directly in the config.php? - Would be more flexible and less code ;-)