[Fix Issue 15719] Add a variable for .form-group's margin-bottom
[Fix Issue 15719] Add a variable for .form-group's margin-bottom

BS3 commits: 8ef21c73aa49a487ffd2e406ae830edb2bf460b5

mozillo/todc-bootstrap,2947721120/googlebt4,todc/todc-bootstrap,todc/todc-bootstrap,up2vs/todc-bootstrap,ztx1491/todc-bootstrap,aaroncnhk/todc-bootstrap,todc/todc-bootstrap,2947721120/todc-bootstrap