add missing license headers, test to makefile (#7711)

scripts/license-check.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+set -e
+IFS=$'\n'
+count=0
+for fileType in GoFiles; do
+    for file in `go list -f $'{{range .GoFiles}}{{$.Dir}}/{{.}}\n{{end}}' "$@"`; do
+        case $file in
+            */utils/lru.go)
+            # Third-party, doesn't require a header.
+            ;;
+        *)
+            if ! grep 'Mattermost, Inc. All Rights Reserved.' $file -q; then
+                >&2 echo "FAIL: $file is missing a license header."
+                ((count++))
+            fi
+        esac
+    done
+done
+if [ $count -eq 0 ]; then
+    exit 0
+fi
+
+if [ $count -gt 1 ]; then
+    >&2 echo "$count files are missing license headers."
+else
+    >&2 echo "$count file is missing a license header."
+fi
+exit 1