Changelog: 6.4.3 release (#19862)

* Changelog: 6.4.3 release

Co-Authored-By: Marcus Efraimsson <marcus.efraimsson@gmail.com>

CHANGELOG.md

+# 6.4.3 (2019-10-16)
+
+### Bug Fixes
+* **Alerting**: All notification channels should send even if one fails to send. [#19807](https://github.com/grafana/grafana/pull/19807), [@jan25](https://github.com/jan25)
+* **AzureMonitor**: Fix slate interference with dropdowns. [#19799](https://github.com/grafana/grafana/pull/19799), [@aocenas](https://github.com/aocenas)
+* **ContextMenu**: make ContextMenu positioning aware of the viewport width. [#19699](https://github.com/grafana/grafana/pull/19699), [@krvajal](https://github.com/krvajal)
+* **DataLinks**: Fix context menu not showing in singlestat-ish visualisations. [#19809](https://github.com/grafana/grafana/pull/19809), [@dprokop](https://github.com/dprokop)
+* **DataLinks**: Fix url field not releasing focus. [#19804](https://github.com/grafana/grafana/pull/19804), [@aocenas](https://github.com/aocenas)
+* **Datasource**: Fixes clicking outside of some query editors required 2 clicks. [#19822](https://github.com/grafana/grafana/pull/19822), [@aocenas](https://github.com/aocenas)
+* **Panels**: Fixes default tab for visualizations without Queries Tab. [#19803](https://github.com/grafana/grafana/pull/19803), [@hugohaggmark](https://github.com/hugohaggmark)
+* **Singlestat**: Fixed issue with mapping null to text. [#19689](https://github.com/grafana/grafana/pull/19689), [@torkelo](https://github.com/torkelo)
+* **@grafana/toolkit**: Don't fail plugin creation when git user.name config is not set. [#19821](https://github.com/grafana/grafana/pull/19821), [@dprokop](https://github.com/dprokop)
+* **@grafana/toolkit**: TSLint line number off by 1. [#19782](https://github.com/grafana/grafana/pull/19782), [@fredwangwang](https://github.com/fredwangwang)
+
 # 6.4.2 (2019-10-08)
 
 ### Bug Fixes