Build: use golangci-lint as a make command (#17739)

* Build: use golangci-lint as a make command

* Since gometalinter was deprecated in favor of golangci-lint so it was
  replaced by it. Responsibilities held by the gometalinter was moved to
  golangci-lint

* There was some changes in implementation (that was also mentioned in
  the code comment) between the tools, which uncovered couple errors
  in the code. Those issues were either solved or disabled by
  the inline comments

* Introduce the golangci-lint config, to make their
  configuration more manageable

* Build: replace backend-lint.sh script with make

pkg/api/pluginproxy/ds_proxy_test.go

@@ -65,7 +65,7 @@ func TestDSRouteRule(t *testing.T) {
 				},
 			}
 
-			setting.SecretKey = "password"
+			setting.SecretKey = "password" //nolint:goconst
 			key, _ := util.Encrypt([]byte("123"), "password")
 
 			ds := &m.DataSource{