Registration form: workflow to receive supervisor letter

[jbenito]

The use case is:

• A user (student) registers in Indico.
• The registration form has a field in which the student will enter the email address of his supervisor.
• Indico sends an email to the supervisor asking for 'a letter' (any kind, e.g. agreement, consent, feedback, etc)
• Supervisor uploads the letter through Indico
• 'Registrars' are notified.