Add SQLite write timeout to reduce "database locked" errors

This allows write requests to queue. I've also documented the one-off change to
enable WAL mode.

conservancy/settings/prod.py

@@ -20,6 +20,13 @@ DATABASES = {
     'default': {
         'NAME': '/var/lib/www/database/conservancy-website.sqlite3',
         'ENGINE': 'django.db.backends.sqlite3',
+        # WAL mode allows concurrent reading and writing. It is enabled permanently with:
+        # sudo -u www-data sqlite3 conservancy-website.sqlite3 'PRAGMA journal_mode=WAL;'
+        'OPTIONS': {
+            # Reduce the incidence of "database locked" errors by allowing write
+            # requests to be queued for some seconds.
+            'timeout': 5,
+        },
     }
 }