alerts: per-support-team webhook; printedparts routes low-stock to a chosen team

Support teams gain a webhookurl (migration 7d29 + API + settings-page field), so
a team is a notification target. send_webhook(url=) lets a caller override the
site default with a team's webhook. Printedparts gains a 'alert support team'
setting (printedparts_alert_supportteamid) + selector on its settings page;
low-stock alerts post to that team's webhook, falling back to the site
alert_webhook_url. Email leg unchanged. Same pattern extends to other alerting
plugins (printers low-toner next).

plugins/printedparts/plugin.py

@@ -139,6 +139,9 @@ class PrintedpartsPlugin(BasePlugin):
             ('printedparts_alert_roleids', '', 'string',
              'Comma-separated role ids; every active member of these roles '
              'receives low-stock alerts'),
+            ('printedparts_alert_supportteamid', '', 'string',
+             'Support team whose webhook receives low-stock alerts; empty uses '
+             'the site alert_webhook_url'),
         ]
         for key, value, valuetype, description in defaults:
             if Setting.get(key) is None: