Create a signal for expired API tokens. This signal gets triggered when an

expired API token is used for authentication.
We also switch from using naive date objects to timezone-aware date objects in

API token related unit tests to ensure that test data can be properly compared

to the API token's timezone-aware date objects.

Created unit tests for the new signal and ran tests in

djblets/webapi/tests/test_signals.py
Manually tested authenticating with expired and non expired tokens, confirmed

  that appropriate notifications were sent and triggered by the signal.