Explore: Query field fixes

Fix misspelled authentication in Auth overview doc

Still have to figure out if we should have templating for aggregation
fields

Don't use unnest in queries for redshift compatibility

Explore: Fix click to filter for recording rule expressions

- recording rule names contain ':'
- include this in the pattern for metric names

Fix megacheck issues

$ gometalinter --vendor --disable=all --enable=megacheck --deadline=10m ./...
pkg/tsdb/elasticsearch/client/client.go:147:13:warning: should use time.Since instead of time.Now().Sub (S1012) (megacheck)
pkg/tsdb/elasticsearch/client/client.go:190:14:warning: should use time.Since instead of time.Now().Sub (S1012) (megacheck)
pkg/tsdb/elasticsearch/client/client.go:218:13:warning: should use time.Since instead of time.Now().Sub (S1012) (megacheck)

$ gometalinter --vendor --disable=all --enable=megacheck --deadline=10m ./...
pkg/plugins/dashboards_updater.go:51:2:warning: 'if err != nil { return err }; return nil' can be simplified to 'return err' (S1013) (megacheck)

$ gometalinter --vendor --disable=all --enable=megacheck --deadline=10m ./...
pkg/services/sqlstore/alert_notification.go:242:3:warning: 'if err != nil { return err }; return nil' can be simplified to 'return err' (S1013) (megacheck)

disable codecov

removes unused files

Fix setting test

ref #11433