Custom meta tag for to track spelling suggestions

For finders where we show spelling suggestions, we want to
track their page view impressions.
First part of the work is in static
alphagov/static#1927

Here we insert the meta tag into the document head for analtyics script
to pick up.

app/views/finders/_spelling_suggestion.html.erb

@@ -1,10 +1,16 @@
+<% # we want an empty string if there are no suggestions on page load %> 
+<% _spelling_suggestion = '' %>
 <% if @spelling_suggestion_presenter.suggestions.any? %>
 <p class="govuk-body">Did you mean
   <% @spelling_suggestion_presenter.suggestions.each do |suggestion| %>
     <%= link_to suggestion[:keywords], suggestion[:link],
       class: "govuk-link govuk-!-font-weight-bold",
       :data => suggestion[:data_attributes]
     %>
+    <% _spelling_suggestion << suggestion[:keywords] %>
   <% end %>
 </p>
-<% end %>
+<% end %>
+<% content_for :head do %>
+  <meta name="govuk:spelling-suggestion" content="<%= _spelling_suggestion %>">
+<% end %>