adding required attribute for example signin form
adding required attribute for example signin form

BS3 commits: a44bd01628d66b92bee3eaef7e4b4f21b5525de6

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