The default token generator can be set via 

settings.DJBLETS_DEFAULT_API_TOKEN_GENERATOR but this currently takes a class,

which isn't safe to import via settings.py. With this change we instead accept

a token generator ID, which is safe and ensures that we're only using token

generators that are registered.

Created new unit tests for getting the default token generator.
Ran unit tests in djblets.secrets.tests.
Ran unit tests in reviewboard.webapi.tests.