Fix link to debates help

This link used to open in a new window, and we accidentally changed that
behavior while refactoring it in commit c2710de5f.

Since we're adding a test for this case, and the Proposals::NewComponent
class is similar, we're adding a test for that class too. In the case of
proposals, we need to sign in a user because the proposals form contains
fields to attach image, that currently rely on a user being signed in.

spec/components/proposals/new_component_spec.rb

@@ -0,0 +1,16 @@
+require "rails_helper"
+
+describe Proposals::NewComponent do
+  before { sign_in(create(:user)) }
+  let(:proposal) { build(:proposal) }
+
+  describe "header" do
+    it "contains a link to open the help page in a new window" do
+      render_inline Proposals::NewComponent.new(proposal)
+
+      page.find("header") do |header|
+        expect(header).to have_css "a[target=_blank]", exact_text: "How do citizen proposals work?"
+      end
+    end
+  end
+end