Extend git commit hook to check for missing schema migrations

scripts/git-hooks/pre-commit

@@ -12,3 +12,5 @@ exec 1>&2
 echo "Validating PEP8 compliance..."
 pycodestyle --ignore=W504,E501 netbox/
 
+echo "Checking for missing migrations..."
+python netbox/manage.py makemigrations --dry-run --check