Merge branch 'diff-aggregate' of https://github.com/oliverpool/grafana into oliverpool-diff-aggregate

closes #7265

fix(browser history): fixes and enhancements to browser history, it now works properly again AND it can restore previous time ranges in dashboards, closes #7259

* test(dashboard): tests that he extractor does not modify original json

* feat(extractor): validate a cloned version of the dashboard json

Enable error and no_data overrides when testing alerting.

* fix(alerting): fixed evaluation for no_value condition, fixes #7244

* feat(alerting): moving null library into grafana, fixing handling on no value / no series

closes #7149

With present postinst script on Centos 7, restart after upgrade is not working with Warning: grafana-server.service changed on disk. Run 'systemctl daemon-reload' to reload units.

So i added systemctl daemon-reload.

Fix typo in tags API documentation example request

`diff = current - first` (when available)

ref #7234