DB: create_add_custom_fields associates options incorrectly

[cdparra]

When creating an optional value for a custom field in a campaign, through the function create_add_custom_fields_option, the option becomes associated to the first custom field that has the same name, and because names are not unique, this can lead to errors. We need a way of telling the function to use the custom field by id or to look among the custom fields of a certain resource space by name.