When adding secondary email, trigger notification

[jbenito]

When adding a secondary email address, Indico should trigger an email to make sure that this email address belongs to that user.
This can be a security problem if we use email to grant access to some info (to check if we do)

[jbenito]

Another examples:

1. If we grant submission, chair, etc rights to somebody that is not on Indico, Indico will send the an email to the guy. If we create an account with that guy's email before he does, we get his credentials.
2. Another one that it is not so critical is to access an abstract (just to view). All the authors have access by email. I need just to create an account with those emails and the access is granted.

[jbenito]

commit: f750c34e95383ce1dd2bbaabbf20c26acac30f00
commit: 47b5cdac04b186d4c189fdd538abd46413a1ce21
commit: 8573121accafe1abf85d831d30b241f914a3f640