Add ShortUUIDField based on shortuuid

A `ShortUUIDField`. Taken from `shortuuid`:

> Often, one needs to use non-sequential IDs in places where users will see them, but the IDs must be as concise and easy to use as possible. shortuuid solves this problem by generating uuids using Python's built-in uuid module and then translating them to base57 using lowercase and uppercase letters and digits, and removing similar-looking characters such as l, 1, I, O and 0.