fix(dashboard): update local sentAt when sending campaign

When sending a campaign for the first time, the `sentAt` field for the
in-memory representation of `Campaign` is still undefined. This causes
"invalid_Date" to appear in the filename of delivery reports.

Fix this by updating the `sentAt` field of the in-memory `Campaign` to
the current date when sending the campaign for the first time.

Fixes issue #1113

frontend/src/components/dashboard/create/util.ts

@@ -17,5 +17,5 @@ export const confirmSendCampaign = async ({
   if (sendRate) {
     sendUserEvent(GA_USER_EVENTS.USE_SEND_RATE, channelType)
   }
-  updateCampaign({ status: Status.Sending })
+  updateCampaign({ status: Status.Sending, sentAt: new Date().toISOString() })
 }