Ignore 'canceled' flows that aren't new users

This fixes https://github.com/mozilla/kpi-dashboard/issues/16, where we were seeing users hit Stage 1 and Stage 4 without the intervening stages (email verification). The scenario is described here: https://github.com/mozilla/browserid/issues/3359. (The dashboard doesn't use new_account at the moment, but we should probably correct it in the data). The logic behind the fix: if the user is prompted to enter a password, this isn't a new user flow.

You can see the results of the fix on kpi-prod-stage, a staging area for dashboard fixes that uses production data. (Same privacy/security settings as production).

@ozten or @shane-tomlinson, a review would be great, no rush