Allow showing individual global bar links

It's possible that we might want to show only one of the Brexit/Covid links
that are on the global bar.

This PR allows this and makes the margin underneath the upper link dependent
on whether there is anything below it.

app/views/notifications/_global_bar.html.erb

@@ -6,14 +6,18 @@
   link_text = false
   link_href = false
 
+  show_coronavirus_link = true
   coronavirus_title = "Coronavirus (COVID-19)"
   coronavirus_href = "/coronavirus"
   coronavirus_subtext = "National lockdown: stay at home"
 
+  show_transition_link = true
   transition_title = "Brexit"
   transition_href = "/transition"
   transition_subtext = "Check what you need to do"
 
+  coronavirus_link_margin_bottom = show_transition_link ? { margin_bottom: 3 } : {}
+
   # Toggles banner being permanently visible
   # If true, banner is always_visible & does not disappear automatically after 3 pageviews
   # Regardless of value, banner is always manually dismissable by users
@@ -66,18 +70,23 @@
         </p>
       <% end %>
 
-      <% if coronavirus_title && coronavirus_href %>
+      <% if show_coronavirus_link || show_transition_link %>
         <div class="global-bar-covid-wrapper">
+      <% end %>
+
+      <% if show_coronavirus_link %>
           <%= render "govuk_publishing_components/components/action_link", {
             text: coronavirus_title,
             href: coronavirus_href,
             subtext: coronavirus_subtext,
             classes: "js-call-to-action",
             light_text: true,
             simple_light: true,
-            margin_bottom: 3
-          } %>
+          }.merge(coronavirus_link_margin_bottom)
+         %>
+      <% end %>
 
+      <% if show_transition_link %>
           <%= render "govuk_publishing_components/components/action_link", {
             text: transition_title,
             href: transition_href,
@@ -86,6 +95,9 @@
             light_text: true,
             simple_light: true
           } %>
+      <% end %>
+
+      <% if show_coronavirus_link || show_transition_link %>
         </div>
       <% end %>
     </div>