add Referer header to logs. This is useful to link datasource requests to dashboard that originated it (#8399)

* support setting the handler attribute in sensu alert notifications

* allow the user to set the source attribute of Sensu notifications

* remove X-Forwarded-* headers added by nginx when proxying data source & plugin requests

* properly handle X-Forwarded-For

fix: s3 image uploads to us-east-1 (us standard) did not return correct image urls so images shared to slack and other services was not correct, fixes #8444

Simple query editor - a text area with a Show Help section.
Validation for empty query and if the time_sec column is missing.

grafana_com: changed name of oauth grafana_net integration (old settings names still work), and updated login button look, closes #8415