Add option to hide name from signups

[PurkkaKoodari]

Currently, names in signups are always public, and the app tells the user to use a pseudonym if necessary. However, usually organizers still want to collect people's names. There are also GDPR issues with publicly showing names.

This also came up in #52, and has come up in recent events in Tietokilta's signup system, where a separate "public name" field was used - definitely not an ideal solution.

Add a checkbox to signups that chooses if the name is shown publicly, otherwise just show "Hidden".