Centers all login-register forms

this is especially necessary for password-reset forms

lms/static/sass/views/_login-register.scss

@@ -101,6 +101,7 @@
 
   background: $white;
   min-height: 100%;
+  margin: 0 auto;
   padding-left: ($baseline/2);
   padding-right: ($baseline/2);
   $third-party-button-height: ($baseline*1.75);