Start tracking the decision task of the data pipeline (#649)

This way we can automate the stop of a running data pipeline during the
rollback process.

infra/taskcluster-hook-pipeline-start.json

@@ -51,7 +51,8 @@
         "retries": 5,
         "routes": [
             "notify.email.release-mgmt-analysis@mozilla.com.on-failed",
-            "notify.irc-channel.#bugbug.on-failed"
+            "notify.irc-channel.#bugbug.on-failed",
+            "index.project.relman.bugbug.data-pipeline-start"
         ],
         "schedulerId": "-",
         "scopes": [