Add E-Mail setup & validation to installation wizzard

[M4LuZ]

Expected Behavior

LS should take steps to ensure that the email setup is done properly during the initial setup phase.
That should include both providing input fields for all related settings, but also a way to validate setup easily by sending a test mail

Current Behavior

New configuration options introduced with #658 are not visible in the first setup.
Also there is no option to trial the settings

Possible Solution

Add related fields.
Add either automatic sending or a separate button to trigger email sending.
Test mail should contain a confirmation link to also ensure that URL setup is correct