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

Merge branch 'update-cloudwatch-extended-stats-check' of https://github.com/apalaniuk/grafana into apalaniuk-update-cloudwatch-extended-stats-check

Remove user form org in single org setup improvement 

13340 complete oauth doc

See,
$ gometalinter --vendor --deadline 6m --disable-all --enable=megacheck ./...
pkg/services/alerting/notifiers/telegram.go:130:2:warning: should check returned error before deferring imageFile.Close() (SA5001) (megacheck)

See,
$ gometalinter --vendor --deadline 6m --disable-all --enable=megacheck ./...
pkg/tsdb/mssql/mssql_test.go:695:33:warning: do not pass a nil Context, even if a function permits it; pass context.TODO if you are unsure about which Context to use (SA1012) (megacheck)
pkg/tsdb/mysql/mysql_test.go:772:32:warning: do not pass a nil Context, even if a function permits it; pass context.TODO if you are unsure about which Context to use (SA1012) (megacheck)
pkg/tsdb/postgres/postgres_test.go:704:33:warning: do not pass a nil Context, even if a function permits it; pass context.TODO if you are unsure about which Context to use (SA1012) (megacheck)

stackdriver docs: metric query editor and annotations

- index page datasource order changed
- image links changed to the image box format
- incorrect sentence fixed

Frontend extensions