Forms: Store all data, Prevent data loss in altered forms and export all forms data in the relevant CSVs

[mariobehling]

If an existing form is changed the question remains what happens to the data that was entered into the form before it was changed? We should handle this similar to Google forms/spreadsheets.

1. Firstly all form data should be kept in the database and it should be added to the exported CSV of attendees, orders and speakers exports.

2. If a custom form field is changed the content in the database field (and the heading in the export CSV) should simply be changed as well.

3. If a custom form field is deleted and no user has entered data yet, then the database field can simply be deleted.

4. If a custom form field is deleted and a user has already entered data, then the database fields are stored, but in the frontend the fields become invisible (and also the answers of the user become invisible). However, in the export CSV these fields should also still show up.