Add method to generate a list of links

A list of links is a very common pattern in the web, and we use it in
many places. Here we're applying it to one of the most simple ones; the
help page.

Generally speaking, I'm not a big fan of helpers, but there are methods
which IMHO qualify as helpers when:

* They do not deal with application objects but mainly strings and
  arrays
* They return text or an HTML tag
* Their logic is simple and splitting it into several methods is not
  necessary

Many Rails helpers, like `tag`, follow these principles.

spec/helpers/link_list_helper_spec.rb

@@ -0,0 +1,46 @@
+require "rails_helper"
+
+describe LinkListHelper do
+  describe "#link_list" do
+    it "returns an empty string with an empty list" do
+      expect(helper.link_list).to eq ""
+    end
+
+    it "returns nothing with a list of nil elements" do
+      expect(helper.link_list(nil, nil)).to eq ""
+    end
+
+    it "generates a list of links" do
+      list = helper.link_list(["Home", "/"], ["Info", "/info"], class: "menu")
+
+      expect(list).to eq '<ul class="menu"><li><a href="/">Home</a></li><li><a href="/info">Info</a></li></ul>'
+      expect(list).to be_html_safe
+    end
+
+    it "accepts options for links" do
+      render helper.link_list(["Home", "/", class: "root"], ["Info", "/info", id: "info"])
+
+      expect(page).to have_css "a", count: 2
+      expect(page).to have_css "a.root", count: 1, exact_text: "Home"
+      expect(page).to have_css "a#info", count: 1, exact_text: "Info"
+    end
+
+    it "ignores nil entries" do
+      render helper.link_list(["Home", "/", class: "root"], nil, ["Info", "/info", id: "info"])
+
+      expect(page).to have_css "a", count: 2
+      expect(page).to have_css "a.root", count: 1, exact_text: "Home"
+      expect(page).to have_css "a#info", count: 1, exact_text: "Info"
+    end
+  end
+
+  attr_reader :content
+
+  def render(content)
+    @content = content
+  end
+
+  def page
+    Capybara::Node::Simple.new(content)
+  end
+end