feat: integrated exercises into lessons

- an exercise can now be added to a lesson
- it is rendered using livecode editor with submit button
- remembers the submitted code and shows the submission time

Issue #90

community/www/courses/learn/index.html

@@ -55,8 +55,14 @@
 {% macro render_section(s) %}
   {% if s.type == "text" %}
     {{ render_section_text(s) }}
-  {% elif s.type == "example" or s.type == "code" or s.type == "exercise" %}
-    {{ LiveCodeEditor(s.name, s.get_latest_code_for_user(), s.type=="exercise", "2 hours ago") }}
+  {% elif s.type == "example" or s.type == "code" %}
+    {{ LiveCodeEditor(s.name,
+          code=s.get_latest_code_for_user(),
+          reset_code=s.contents,
+          is_exercise=False)
+    }}
+  {% elif s.type == "exercise" %}
+    {{ widgets.Exercise(exercise=s.get_exercise())}}
   {% else %}
     <div>Unknown section type: {{s.type}}</div>
   {% endif %}