Ability for ordinary users to add domains

[wioxjk]

It would be great if ordinary users could add domains themselfes, instead of involving the admin.

[cemilf]

This feature would be definitely nice for users because every time our clients need to add a domain they need to send an e-mail or open a support ticket.

Also a domain quota option can be added to users so that we can limit the number of domains a user can manage when required.

[wioxjk]

@loewexy

How about open a liberapay/opencollective account so we can chip in some pizza-money for you?