Make template usage on dashboard easier to scan

The dashboard looked a bit table-y. This commit makes four main changes:

- show a bar chart (drawn in CSS) for template usage (only shown if
  you’ve used more than one template recently)
- only break down template usage by template name, not template type
  (because that’s happening with the big numbers)
- change the style of the ‘show more’ links under each section so that
  they are all consistent, and a little less busy (one less keyline)
- remove the ‘recent templates‘ title so that the first two sections of
  the page group under ‘in the last 7 days’

app/templates/views/dashboard/dashboard.html

@@ -8,6 +8,8 @@
 
   <div class="dashboard">
 
+    <h1 class="visuallyhidden">Dashboard</h1>
+
     {% if not templates and current_user.has_permissions(['send_texts', 'send_emails', 'send_letters'], any_=True) %}
       {% include 'views/dashboard/get-started.html' %}
     {% elif current_user.has_permissions([
@@ -20,9 +22,9 @@
       {% include 'views/dashboard/no-permissions-banner.html' %}
     {% endif %}
 
-    <h1 class="heading-medium">
+    <h2 class="heading-medium">
       In the last 7 days
-    </h1>
+    </h2>
     {% include 'views/dashboard/today.html' %}
 
   </div>