Ensure Google events sync has a channel token

[lmickh]

I cannot find where we enable the Google channel token header in the webhooks or validate it. The option is off by default. Do we know that we are setting it? If not, we should enable it since it is effectively the password to the corresponding channel id.

Related code:
https://github.com/closeio/nylas/blob/n17/inbox/events/remote_sync.py
https://github.com/closeio/nylas/blob/n17/inbox/events/google.py

Information about the id and token headers can be found here:
https://developers.google.com/calendar/v3/push#understanding-the-notification-message-format