Add E-Mail Verification and possibility for user to enter an e-mail address

Users should be able to store additional e-mail addresses with their identity. Implement a check to verify e-mail adresses.

added enhancement label

mentioned in commit 1839d5e2

mentioned in commit 9e9ae459

mentioned in commit 2257f745

assigned to @michael.simon

changed milestone to %2.8.1

Close for now. Open new issues for possible bugs.

closed