Reintroduce prefers_digital_correspondence field to person and add spec

Refs: hitobito/hitobito#2846

app/views/contactable/_address_fields_pbs.html.haml

@@ -0,0 +1,3 @@
+- if entry.is_a?(Person)
+  = field_set_tag do
+    = f.labeled_boolean_field :prefers_digital_correspondence

spec/views/people/_fields.html.haml_spec.rb

@@ -0,0 +1,29 @@
+require 'spec_helper'
+
+describe 'people/_form.html.haml' do
+  let(:group) { person.primary_group }
+  let(:person) do
+    people(:bulei)
+  end
+  let(:stubs) {
+    { path_args: [group, person], model_class: Person, entry: person }
+  }
+
+  before do
+    allow(view).to receive_messages(stubs)
+    allow(controller).to receive_messages(current_user: person)
+  end
+
+  let(:dom) do
+    render partial: "people/form"
+    Capybara::Node::Simple.new(rendered)
+  end
+
+  describe "prefers_digital_correspondence" do
+    it "renders field" do
+      expect(dom).to have_css("label", text: "Digitale Korrespondenz bevorzugt")
+      expect(dom).to have_css("input[name='person[prefers_digital_correspondence]']")
+    end
+  end
+
+end