printedparts: show revision at kiosk; low-stock email uses gage tag + item link

Kiosk now displays the scanned revision (badge, quantity, and done screens) so
the operator sees which revision they checked out - it was already recorded on
the take, just not shown. Low-stock email now uses the gage lab tag (was the
internal item code) and links to the item page when site_base_url is set (new
core setting, category email; emails have no request context to derive the URL).

shopdb/core/api/settings.py

@@ -830,6 +830,14 @@ def build_default_settings():
             'category': 'email',
             'description': 'Default email recipients for alerts (comma-separated)'
         },
+        {
+            'key': 'site_base_url',
+            'value': '',
+            'valuetype': 'string',
+            'category': 'email',
+            'description': 'Public base URL of this site (e.g. '
+                           'https://host/ops), used to build links in emails'
+        },
         # Audit log settings
         {
             'key': 'audit_retention_days',