Refactor to make it logic clearer

The logic around what gets shown on the uploads page was hard to follow.
This commit makes three changes to hopefully make it easier:
- remove the passing-around of a string containing the contents of a
  rendered partial
- encapsulate everything in one `show_scheduled_jobs` variable, rather
  than mixing between `scheduled_jobs` and
  `current_service.scheduled_jobs`
- adds a comment to explain why we still render `_jobs.html` even when
  we know we have no jobs

app/templates/views/jobs/jobs.html

@@ -9,8 +9,19 @@
 {% block maincolumn_content %}
     <h1 class="heading-large">Uploads</h1>
     <div class="dashboard">
-      {{ scheduled_jobs|safe }}
-      {% if jobs or not current_service.scheduled_jobs %}
+      {% if show_scheduled_jobs %}
+        {% with hide_heading = True %}
+          {% include 'views/dashboard/_upcoming.html' %}
+        {% endwith %}
+      {% endif %}
+      {% if jobs %}
+        {% include 'views/dashboard/_jobs.html' %}
+      {% endif %}
+      {% if not jobs and not show_scheduled_jobs %}
+        {#
+          `_jobs.html` will show the ‘You have no jobs’ message when
+          passed an empty list of jobs
+        #}
         {% include 'views/dashboard/_jobs.html' %}
       {% endif %}
       {{ previous_next_navigation(prev_page, next_page) }}