add clear cache platform admin page

a form that allows you to clear entries from the cache for all of
either users, services or templates. It'll tell you the largest amount
of keys deleted, since there are multiple keys associated with each
model.

app/templates/views/platform-admin/_base_template.html

@@ -24,6 +24,7 @@
           ('Email Complaints', url_for('main.platform_admin_list_complaints')),
           ('Returned letters', url_for('main.platform_admin_returned_letters')),
           ('Letter validation preview', url_for('main.platform_admin_letter_validation_preview')),
+          ('Clear cache', url_for('main.clear_cache')),
         ] %}
           <li>
             <a href="{{ url }}">