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