Make mailers as similar as possible
Create a common mailer
Make the mailer a service and implement it in all modules
Use dependency injection to inject the database into the mailer service
Remove debugging code
Remove Backend and Frontend mailers since they aren't compatible with the parent anymore
Add Mailer as a service in the changelog and upgrade guide
Call the mailer service in a nicer way
Make the mailer code clearer
Remove some useless comments
Use the common mailer in the sendQueuedEmails cronjob
Add credit
Remove unused use statement
Merge pull request #697 from WouterSioen/mailer-as-a-service

Make the mailer a service. Fix #691
Fixed mistake in upgrade guide