Add release notes for 1.2.6 (#5011)

* Add release notes for 1.2.6

* Update content/en/boilerplates/notes/1.2.6.md

content/en/about/notes/1.2.6/index.md

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+---
+title: Istio 1.2.6
+publishdate: 2019-09-17
+icon: notes
+release: 1.2.6
+---
+
+This release includes bug fixes to improve robustness.  This release note describes what's different between Istio 1.2.5 and Istio 1.2.6.
+
+{{< relnote >}}

content/en/blog/2019/announcing-1.2.6/index.md

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+---
+title: Announcing Istio 1.2.6
+description: Istio 1.2.6 patch release.
+publishdate: 2019-09-17
+attribution: The Istio Team
+release: 1.2.6
+---
+
+We're pleased to announce the availability of Istio 1.2.6. Please see below for what's changed.
+
+{{< relnote >}}

content/en/boilerplates/notes/1.2.6.md

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+## Bug fixes
+
+- Fix `redisquota` inconsistency in regards to `memquota` counting ([Issue 15543](https://github.com/istio/istio/issues/15543)).
+- Fix an Envoy crash introduced in Istio 1.2.5 ([Issue 16357](https://github.com/istio/istio/issues/16357)).
+- Fix Citadel health check broken in the context of plugin certs (with intermediate certs) ([Issue 16593](https://github.com/istio/istio/issues/16593)).
+- Fix Stackdriver Mixer Adapter error log verbosity ([Issue 16782](https://github.com/istio/istio/issues/16782)).
+- Fix a bug where the service account map would be erased for service hostnames with more than one port.
+- Fix incorrect `filterChainMatch` wildcard hosts duplication produced by Pilot ([Issue 16573](https://github.com/istio/istio/issues/16573)).
+
+## Small enhancements
+
+- Expose `sidecarToTelemetrySessionAffinity` (required for Mixer V1) when it talks to services like Stackdriver. ([Issue 16862](https://github.com/istio/istio/issues/16862)).
+- Expose `HTTP/2` window size settings as Pilot environment variables ([Issue 17117](https://github.com/istio/istio/issues/17117)).