archweb: allow re-using the postgresql connection

Keep postgresql connections around for 5 minutes this avoids expensive
authentication requests.

roles/archweb/templates/local_settings.py.j2

@@ -23,6 +23,7 @@ DATABASES = {
     'default': {
         'ENGINE'  : 'django.db.backends.postgresql_psycopg2',
         'PORT'    : 5432,
+        'CONN_MAX_AGE' : 600,
 {% if archweb_db_host != 'localhost' %}
         'HOST'    : '{{ archweb_db_host }}',
 {% endif %}