Use 'SubGRB' to identify subthreshold GRBs

Previously was using a parameter 'NoticeType' to idenfity sub-threshold GRBs. However checking a sub-threshold GRB VOEvent this field does not exist: https://sentry.io/organizations/ligo-caltech/issues/998630013/events/da05309eed5645e9aafd582186965478/#extra

Since we are switching to have a separate search for sub-threshold GRBs (search=='SubGRB') we can instead use this. I changed test_raven_without_initial_circular to have a sub-threshold GRB so that we can see that this change works.

ligo/followup_advocate/test/data/E5678/event.json

@@ -38,7 +38,7 @@
     "far": null,
     "instruments": "",
     "labels": [],
-    "search": "GRB",
+    "search": "SubGRB",
     "nevents": null,
     "superevent": "S5678",
     "submitter": "emfollow",