Consider changing all calls to `deliver_now` to `deliver_later`

[floehopper]

From #98 (comment):

@chrislo I wonder whether it's worth changing the email sent in RegistrationsController#send_email_verification to be sent with ActiveJob, i.e. using deliver_later. That way if there's an exception, they won't see an error page. In fact I wonder whether all emails should be sent with deliver_later...?

[chrislo]

Yes, I can't see any downsides to doing that. I suppose if we have a very high volume of jobs and a single job queue then calling deliver_later for things like password resets might not be optimal as they could arrive late. But we don't have that issue at the moment and it's probably better solved by using queues with different priorities.