Move user approval notifications from the model to workers

Instead of sending emails from after_create and after_update callbacks in
User, now there are a few resque workers that detect when approval emails
need to be sent and send them.

This fix the fact that sometimes the worker called to send an email was
running even before the user was persisted to the database, generating an
error in the worker.

refs #1403
Skip resque-scheduler tests broken by adc28067

refs #1403
Merge branch 'master' into 1310-admins-accepting-join-requests

Conflicts:
	db/schema.rb