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.

app/helpers/link_list_helper.rb

@@ -0,0 +1,13 @@
+module LinkListHelper
+  def link_list(*links, **options)
+    return "" if links.compact.empty?
+
+    tag.ul(options) do
+      safe_join(links.compact.map do |text, url, **link_options|
+        tag.li do
+          link_to text, url, link_options
+        end
+      end)
+    end
+  end
+end