printedparts stage 13: pick alert recipients from shopdb users

Contract 0.13.0 puts the User model on the plugin surface. The
settings page gains a checkbox picker over the user list; selected
users receive low-stock alerts at their account email, merged and
deduped with the free-text address list, inactive accounts skipped,
site alert_recipients still the fallback when both are empty.

plugins/printedparts/plugin.py

@@ -133,6 +133,9 @@ class PrintedpartsPlugin(BasePlugin):
             ('printedparts_alert_email', '', 'string',
              'Comma-separated low-stock alert recipients; empty uses the '
              'site alert_recipients'),
+            ('printedparts_alert_userids', '', 'string',
+             'Comma-separated shopdb user ids whose account emails receive '
+             'low-stock alerts'),
         ]
         for key, value, valuetype, description in defaults:
             if Setting.get(key) is None: