Hide `help-block` margins until `form-group` gets `has-error`

[derekm]

help-blocks in the bootstrap-decorator consume whitespace even when error states are disabled. In issue #214, user @plong0 points out that it would be nice to hide help-blocks until there are errors.

This is an issue because help-block has top & bottom margins that consume space and tend to make form inputs appear "too far" apart.

I used a version of plong0's CSS, which is:

.form-group .help-block {
    display: none;
    visibility: hidden;
}

.form-group.has-error .help-block {
    display: block;
    visibility: visible;
}

Ultimately, bootstrap-decorator should be improved so error messages don't cause the form elements to redraw themselves and jump up & down on the screen.