Airdrop feature

1. When requesting an opt-in inside the optional data parameter a field airdop can be specified with an amount.
2. If this field is defined and bigger then 0 the confirmation email sends publicKey and privateKey together with the confirmation Email. For that two new template params need to be defined ("privateKey", "publicKey") where those keys are send to the recipient of the email.
3. When the email gets confirmed on Alice dApp, Alice send's the amount (of the airdrop field) to the associated publicKey.

A test must be written.