the button of signup can be used
change 'xianyin' to 'is_there_any_activity';change 'create_activity_name' to 'activity_names'
add some comments
write the judgment function
write the storage function
change the state of button(start_or_end) of sign_up.html
reset some arrays to object in the create_activity.html
change the data type in the activity_list.html
separate some functions from the .js and the start_or_end button of sign_up.html can be used
add some comments to the function