Currently when a WebAPI token is created, we always send an e-mail

notifying the user of its creation. However, this is not always ideal.

Some extensions and/or integrations require a WebAPI token in order to

communicate with Review Board from some external service (such as

CircleCI). In this case, we do not need to bother the user with a

notification that the token was created.
Djblets now has support for token update and create signals (via

/r/9275). We use these signals to determine if the token generated was

auto-generated. The notifications module has been modified to listen

to the new signals (instead of the generic post_save signal it was

listening to before) and not send e-mails when the token in question

is auto-generated.

Ran unit tests.