Align form labels' bottom spacing with design specs

We recently noticed that our form labels currently have 8px bottom spacing while it should be 4px according to the design specs.

Let's make sure the implementation is aligned with Pajamas.

Things to look into:

• .col-form-label bottom padding: src/components/base/form/form_group/form_group.scss#L9
• Bootstrap's $label-margin-bottom variable: scss/_reboot.scss#L299

/cc @ameliabauerly @jeldergl