This change updates the APITokenResource according to the new WebAPIToken

fields addded as part of our move to API Tokens v2. The API now creates tokens

in the vendor checksum format rather than the legacy SHA1 format. Users can also

invalidate tokens via the API.
This change also corrects some PUT, POST and DELETE tests for for the

APITokenResource. These tests were all calling self.api_get instead of the

appropriate HTTP method.
It also updates test utilities that are related to WebAPITokens.

Ran unit tests in reviewboard/webapi/tests.