migrate site id to varchar instead of int

backend/apps/sync/models.py

@@ -20,7 +20,7 @@ class ActiveSite(models.Model):
         blank=True,
         default="",
         db_index=True,
-        help_text="Farm-location Site.site_id (slug) from the registering dashboard",
+        help_text="Farm-location Site.site_id (slug); equals HQ sites.site_id when claimed",
     )
     api_base_url = models.URLField(
         max_length=255,