Add alerting
Implement alerting (#9) * #8: Create manager command to send high percent_used alerts * Fix typo * Move env vars into settings.py * Track recently sent alerts by limit name * Clean up LIMIT_ALERT_PERCENTAGE; add imports
Implement alerting (#9) * #8: Create manager command to send high percent_used alerts * Fix typo * Move env vars into settings.py * Track recently sent alerts by limit name * Clean up LIMIT_ALERT_PERCENTAGE; add imports