Currently, token generation is baked into 

djblets.webapi.managers.WebAPITokenManager. With our move to API Tokens v2,

we don't want token generation to be tightly coupled with Web API tokens. We

want tokens to be usable outside of just the API.
This change moves token generation out of WebAPITokenManager by making use of

token generators instead of directly generating tokens in WebAPITokenManager.

To do this we add two new parameters to WebAPITokenManager, 

token_generator_id to decide what token generator to use and token_info to

pass along information needed for token generation to the token generator.

Updated WebAPITokenManager's unit tests to account for the use

  of token generators.
Ran all unit tests in djblets.webapi.tests with success.