Extract component to render the date of birth

We reduce code duplication thanks to that, and make it easier
to change this field.

Note that there was one place where the "16.years" value was
hardcoded. We're moving the test for this case to the
component and changing it so the test doesn't use the default
age.

We're also removing the redundant helper method that had the
same code as a method in the User class which is called
everywhere else.

spec/components/shared/date_of_birth_field_component_spec.rb

@@ -0,0 +1,26 @@
+require "rails_helper"
+
+describe Shared::DateOfBirthFieldComponent do
+  before do
+    dummy_model = Class.new do
+      include ActiveModel::Model
+      attr_accessor :date_of_birth
+    end
+
+    stub_const("DummyModel", dummy_model)
+  end
+
+  let(:form) { ConsulFormBuilder.new(:dummy, DummyModel.new, ApplicationController.new.view_context, {}) }
+  let(:component) { Shared::DateOfBirthFieldComponent.new(form) }
+
+  it "uses the minimum required age as the latest date by default" do
+    Setting["min_age_to_participate"] = 13
+
+    travel_to "2015-07-15" do
+      render_inline component
+
+      expect(page).to have_select with_options: [2002, 2001, 2000, 1999]
+      expect(page).not_to have_select with_options: [2003]
+    end
+  end
+end