Fixes #13629

Alerting: Propagate alert validation issues to the api/user

Fix grammar in log message

[skip ci]

[skip ci]

Make sure to add all variable nodes to directed acyclic graph (dag) before linking variables

Remove os.Kill as it cannot be trapped

Fix graph render blocked by annotations issue

Removed reference to plugin_api.md (SDK Readme) 

Adding time clock

fix for graph time formating for Last 24h ranges

Removed reference to plugin_api.md (SDK Readme) on the development page as the file that it points to no longer exists. This addresses an open issue on the grafana.org repo

See,
$ gometalinter --vendor --deadline 10m --disable-all --enable=megacheck ./...
pkg/cmd/grafana-server/main.go:103:42:warning: os.Kill cannot be trapped (did you mean syscall.SIGTERM?) (SA1016) (megacheck)

changed to plain errors further down the alerting validation model so error did not get double wrapping in ValidationError

alerting: propagate alert validation issues to the user instead of just 'invalid alert data' message

Fix trivial megacheck issues

fix: label values regex for single letter labels

Typos in plugin development documentation

See,
$ gometalinter --vendor --deadline 10m --disable-all --enable=megacheck ./...
pkg/services/sqlstore/user_auth_test.go:28:18:warning: this result of append is never used, except maybe in other appends (SA4010) (megacheck)

Closes: #13641

updated jest to 23.10

Adds backend hooks service so extensions can modify index data

Stackdriver: Prevent filter from crashing

minor setting refactorings