Set editable flag on attendees' data

[cbellone]

related to #645, but specific to one (or more) additional info.

It can be useful in case the organizers want to let attendees modify an option (e.g. T-Shirt size) up to a certain date/time.

The "read only" flag should have an impact only on existing registration. So if new attendees #, they must see and fill the options the first time.