Customizable e-mail templates

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It would be cool to be able to customize the e-mail template that is sent to users after creating a new account. This should be an admin option.

Based on the discussion marketing_monthly_release#1 (comment 4059921)

If we decide to implement this, we will need it for 8.6.

cc @EmilyKyle @rspeicher @DouweM @JobV @sytses @nearlythere @hanktaylor @ashleys