Currently, Djblets can only generate SHA1 tokens and token generation is

baked into djblets.webapi.managers.WebAPITokenManager. As part of our move to

using an improved token format (API Tokens v2), we need a way to generate

different types of tokens.
This change introduces token generators, which handle the logic for token

generation and allows for multiple ways of generating tokens. There are three

classes implemented:

1. A base class.

2. A class for generating our legacy SHA1 tokens.

3. A class for generating our new token format that has a vendor prefix and

   checksum.
This also adds a registry for the token generators.

Created unit tests for the token generators.
Ran all unit tests in djblets.secrets.tests.