[v3] Add padding to `navbar-fixed-*` on modal open

Created by: iamandrewluca

When a modal is opened, body gets some padding if has vertical scrollbars. Fixed elements on other side don't get this padding, when body has modal-open and it's overflow is hidden. This will add same amount of padding to fixed elements, if there are vertical scrollbars. This issue also was addressed in v4, and was fixed and created another one 😄 . I hope this fix will get into bootstrap-sass

• test added padding
• test removed padding

Fixes: #27330 (closed)