Update process-notification - too many requests error

**User Story**
GCNotify has an API rate limit of 1000 calls per minute, rolling timeframe.
Every minute the `process-notification` job creates a new job to send requests up to the polling limit.
Other jobs (like the SIN response) generates more than 1000 email notifications when it is finished.
When we're over 1000 emails, the second `process-notification` job starts and we very quickly get a 529 error for too many requests.
When these error, an email notification is sent to RequestIT via Sysdig. Currently, we're receiving this error every day.
Too many error notifications when _no error occurs_ can lead to extra work and "alert noise" which can delay actual errors from being seen.

**Acceptance Criteria**
- [ ] Update the process-notification to only allow one active job at a time.