Change the order of assertion generation to be after the user is asked "is this your computer?"

* This fixes a problem where it is possible that a user takes more than 2 minutes to answer, making the already generated assertion invalid.
* A lot of general cleanup of the state machine.
* Push all post email selection flows to a new state, "email_valid_and_ready" which decides whether to ask the user the computer ownership status.
* Assertions are generated after the "is this your computer" is asked.
* doEmailChosen is replaced with doGenerateAssertion - there were multiple calls to generate an assertion, not very DRY.
* Fix a problem in network.js where a user who completes user registration should be marked as authenticated.
* rename email_chosen.js to generate_assertion.js to clarify its purpose.

issue #1460