Installer: Add form validation on setup process 4/5

[YaWK]

In step 4/5, when the future admin is called to enter email adress, admin user and password, there should be form validation on following fields:

• Email Address,
• Admin Username
• Password (+ repeat check)

This ensures, that setup will not collapse in this late stage due typo errors or some other sort of lack.

[YaWK]

Form validation on Step 4/5: email, username and password will be checked before submit.