Abuse prevention in user registration

Since no email confirmation is required (it wasn't when it was mandatory) and every user gets a quota of storage space by default, it makes sense to prevent possible abuse. We should secure this further.

One possibility is captchas. Some validate the user in the background and only require a direct user entry if there is an increased probability of abuse. This is a possible setting option for Google Captcha, for example.

In instances where an email is mandatory, this should also be confirmed.