Add 2, 3 and 6 hours silence options (#1822)

# What this PR does
This PR adds additional silence options in the UI. Currently we only
have 1 hour, 4 hours and 12 hours silence options. I think it's worth it
to have finer silence options.

## Which issue(s) this PR fixes
No issue ticket but I can create one.

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

CHANGELOG.md

@@ -5,6 +5,12 @@ All notable changes to this project will be documented in this file.
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 
+## Unreleased
+
+### Added
+
+- Add 2, 3 and 6 hours silence options
+
 ## v1.2.15 (2023-04-24)
 
 ### Fixed

engine/apps/alerts/models/alert_group.py

@@ -241,7 +241,10 @@ class AlertGroup(AlertGroupSlackRenderingMixin, EscalationSnapshotMixin, models.
     SILENCE_DELAY_OPTIONS = (
         (1800, "30 minutes"),
         (3600, "1 hour"),
+        (7200, "2 hours"),
+        (10800, "3 hours"),
         (14400, "4 hours"),
+        (21600, "6 hours"),
         (43200, "12 hours"),
         (57600, "16 hours"),
         (72000, "20 hours"),