This change introduces a concept of deprecated token generators and allows API

tokens to be considered as deprecated if they were generated by a deprecated

token generator. The deprecated token generators can be set via the

DJBLETS_DEPRECATED_TOKEN_GENERATORS setting.
Now when a deprecated token is used for authentication, a deprecation notice

will be sent via a response header. Previously, the base WebAPIResource

class would include headers set by authentication backends in the response

only when the authentication failed. For the purposes of including

deprecation notices in successful responses, we modified the WebAPIResource

to always include any headers set by authentication backends.

Ran all unit tests
Made requests using deprecated and non deprecated tokens to some

  Reviewboard API endpoints (specifically GET and POST requests to

/review-requests/id/reviews/)
Ran all webapi tests in the Reviewboard test suite.