Send welcome SMS to users after signup

[alexanderhoset]

After a user is signed up send them a welcome message defined in admin as a feedback message

This can be done by listening to "our own" user management events: DataCollectorAdded and SaffUserAdded (More events can be added as #354 splits SaffUser to more roles )

After message is sent MobilePhoneNumberConfirmed on the new user is set to true

notes:

• Default values for automatic reply messages #303 in admin is not done, so for now set a random message .
• The SMS gateway isn't implemented yet, I suggest we create an abstraction that we later integrate with the SMS gateway.

[karolikl]

@Davlind, can you confirm that this issue can be closed?

[Davlind]

Yes, it can be closed. Though, we should ensure that @alexanderhoset notes above have corresponding backlog items, as they're still valid