Add trigger integration (i.e. send users mails on their birthdays) - D7 release blocker

Sub-tasks that need to be done here:

• Basic trigger support
• Reasonable defaults: Have a "Happy birthday mail" action
• Delete triggers when fields are deleted
• Instance setting: Allow users to opt out of triggers
• Make tokens available for the mail (birthday, age, ...)

Please edit this issue summary in order to add any further details as you see fit.