Log account creation with AddNewAccount, impression with UserCreateForm

Instead of conditionally logging event, make token optional on the
server-side.

Bump schemas:

* TrackedPageContentSaveComplete
* SignupExpAccountCreationImpression
* SignupExpAccountCreationComplete

for doc changes, and token being made optional.

Bug: 65352
Change-Id: I843ed9d4cbab221c0cb664f6820ab9139ced94e5
(cherry picked from commit 9b39a555615fd9e27ea7385e23d723f5df7262fa)