* Don't set saw_onboarding automatically
This should be set by the onboarding checkbox update (user accepted
terms of service and code of conduct).
The issue is a PATCH request to track the last seen page is sent on
the first page, _before_ the user consents to the terms via the
checkboxes.
This issue is masked by a 422 unprocessable entity response when we
don't get a username in the patch request (next commit).
* Only validate username on the username page
Every other "last page seen" dialog was returning a 422 because the
username is only submitted in the 3rd dialog "build your profile".
Only validate the username field when it's submitted, process other
onboarding updates normally.
* Remove failing test
Since the onboarding_update action no longer sets saw
onboarding (since we send the patch before they've accepted the code
of conduct), don't test that we do.
There's a parallel test for the onboarding checkbox update later in
this file that covers the checkboxes have been accepted.
* overwrite instead of merging user_params
We now initialize it empty, no need to merge params here.
085c566a7b