Missing mobile phone number field for sign-up forms

[Former user]

When creating a sign-up form, there is no standard field for entering mobile phone numbers. This would be useful to have to keep in contact with participants, e.g. when they're late for an event.

[Martijn Luinstra]

Add phone field to sign-up. Fixes #107

→ <<cset 1180b9a1f083>>