Replace shortid with nanoid

[vrizo]

Hi!

Please consider replacing shortid with nanoid. There are no significant changes for users, but there are some critical benefits:

• shortid is not maintained anymore (and issue tracker has many important issues);
• shortid has a problem with a random generator (issue) and ID uniformity (issue). nanoid uses hardware random generator and has special uniformity tests (docs);
• shortid has a problem with - symbol at the end of URL on some Android apps (issue). nanoid uses safe ~ instead of -;
• And finally, nanoid is 10x times faster than shortid.

What do you think about this, @mcollina?

Thanks!