The `tabId` of the message sender is now added to the middleware
request object. This step is omitted if the `tabId` is not provided.
This is done early in the middleware stack, so the `tabId` should be
available for any subsequent middleware to use.
The Onboarding Middleware has also been modified to get the `tabId`
from the request object, rather than expecting it as a parameter upon
creation.
This refactor will enable further uses of the `tabId`.
