The initial followup_day1 email confirms that the new user account
has been successfully created and should be sent to the user
independently of an organization's setting for send_welcome_emails.
Here we separate out the followup_day1 email into a separate function
from enqueue_welcome_emails and create a helper function for setting
the shared welcome email sender information.
The followup_day1 email is still a scheduled email so that the initial
account creation and log-in process for the user remains unchanged.
Fixes#25268.
3dfdbbc775