Initial work on shorter links
Use a route instead of s.php and convert tokens asap
Adapt token size to new generation and add an option for free length specification

Minimum size is 3 = 46656 tokens, maximum size is 64, which is what our DB allows.
Change config.sample entry and use MediumSecurity
Use base62 and remove configuration options for link length
Readd TOKEN_LENGTH
Match \OC_Helper::linkToPublic() to new links and fit unittests
Merge pull request #10755 from owncloud/shorter_sharing_links

Shorter sharing links