User attributes not clear

[Matthijsy]

Currently the user fields are a bit of a mess. It is not clear when they are required and to which 'category' for the frontend they belong. I hope this issue makes this clear and makes it possible to implement this.

Column	Group	viewable	create by board	create by user	required	default
id	technical	always	no	no	yes
username	technical	always	no	no	yes
email	general	user_setting	yes	yes	yes
first_name	general	always	yes	yes	yes
last_name	general	always	yes	yes	yes
last_name_prefix	general	always	yes	yes	yes
birthday	general	user_setting	yes	yes	yes
address	general	user_setting	yes	yes	yes
postcode	general	user_setting	yes	yes	yes
city	general	user_setting	yes	yes	yes
phone_number	general	user_setting	yes	yes	yes
study	general	user_setting	yes	yes	no
start_study	general	user_setting	yes	yes	no
food_preferences	general	user_settings	yes	yes	no
vegetarian	general	user_settings	yes	yes	yes
emergency_contact	protected	user/board	yes	yes	no
emergency_number	protected	user/board	yes	yes	no
picture_publication_preference	privacy	user/board	yes	yes	yes	always_aks
ifes_data_sharing_preference	privacy	user/board	no	yes	is_activated	null
info_in_almanak	privacy	user/board	no	yes	is_activated	null
almanak_subscription_preference	privacy	user/board	yes	yes	yes	paper
digtus_subscription_preference	privacy	user/board	yes	yes	yes	paper
user_details_sharing_preference	privacy	user/board	yes	yes	is_activated	null

The main difference with the current implementation is that some fields become required as soon as the user is activated.

Another option to fix this issue is to give everything a default and add a boolean which keeps track if the user should see the 'privacy check popup'.