Create registration template component

[MaHaWo]

• make the AbstractDataInput component more abstract such that accepts different components for its items
• add AbstractRegistrationForm for registering new users.
• add a page for child registration using the same form as for user registration.
• create new pages to have a user registration flow to click through

[MaHaWo]

@lkeegan do you want to have another look?

• fixed labels
• made pages into components
• augmented login component with the usual bells and whistles (forgot password? not registered yet? remember me?)
• removed nav buttons from forms
• didn´t refator abstract components yet because that needs more insight from functionality, as discussed.

[lkeegan]

lgtm!