Modal open causes fixed content to shift

Created by: eddywashere

I believe there is an issue with opening modals that causes fixed content to be shifted.

Here's an example http://codepen.io/eddywashere/pen/ZpXrxQ

I'm on OSX, 10.11.5, Chrome 53 and have System Preferences > General > Show scroll bars: always.

I believe this is sort of tracked in the Modal section of v4 issue https://github.com/twbs/bootstrap/pull/17021, but does not contain an active issue to track it.

Finally fix that shifting content bug by updating the modal.js to not adjust body padding, but rather the padding on a specific set of classes.

I found an issue that was closed because it was part of v3, https://github.com/twbs/bootstrap/issues/14040#issuecomment-244817197.