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
Ah, I just had a look into #10642 and
```php
\OC::$server->getSecureRandom()->getLowStrengthGenerator()->generate($tokenLength, '01234567890abcdefghijklmnopqrstuvwxyz');
```
seems like what I need. I'll implement that as soon as #10642 is merged and will remove my base_convert stuff afterwards.

This means I'll need to do some recalculating on the math and `$tokenLength` levels though