margin-bottom for .form-group

Created by: Yahasana

replace margin-bottom: 15px; with margin-bottom: @line-height-computed; for .form-group, which now is inconsistent with .table's

usecase

<h4>hello foo</h4>
<div class="form-group"></div>
                                   <-------
<h4>matrix checkbox fields</h4>            |
<table class="table"></table>              +----------- margin of these should be the same
                                   <-------
<h4>another more</h4>
<div class="form-group"></div>