2.3.0

• [CHANGE] Webhook accepts only POST requests now
• [CHANGE] Environment variable ALERTMONITOR_JOURNAL_SIZE is now obsolete, and it is replaced by ALERTMONITOR_DATA_RETENTION_DAYS
• [FEATURE] Introducing MongoDB. Webhook messages and journal of alerts is stored in MongoDb (if enabled, otherwise in-memory storage is used)
• [FEATURE] Warning messages, in Statistics view (aka. Alertmonitor's internal alarms)
• [FEATURE] Separate alert.log only for alerts
• [ENHANCEMENT] Redesigned synchronization procedure
• [ENHANCEMENT] Added metric alertmonitor_psync_success which returns 1 if last psync was successful.
• [ENHANCEMENT] Added metrics for database operations: alertmonitor_db_inserts_total, alertmonitor_db_queries_total,
  alertmonitor_db_updates_total, alertmonitor_db_deletes_total, alertmonitor_db_failures_total
• [ENHANCEMENT] Added age parameter to alert to show difference between first and clear timestamp.
• [ENHANCEMENT] Added buttons for easier configuration of time intervals in query ranges.
• [ENHANCEMENT] Added hashes (#) in front of tags to emphasize their meaning
• [ENHANCEMENT] Added ALERTMONITOR_HTTP_CLIENT_READ_TIMEOUT_SEC environment variable to set the
  HTTP client read timeout. Also, configurable in Configuration view.
• [ENHANCEMENT] Added prometheusId which consists of a set of labels, that identify this prometheus server.
  Which labels denote to the ID are configurable with environment variable ALERTMONITOR_PROMETHEUS_ID_LABELS.
  Example: cluster, region, monitor.
• [ENHANCEMENT] Improved error handling with Prometheus API
• [ENHANCEMENT] A lot of refactoring
• [BUG] Fixed (#18)
• [BUG] Bugfix (#18)