Update Holder.js to v2.8.2.

[ci skip]

We want to sum two numbers, not concatenate their stringifications.
Closes #17457 by merging a tweaked version of it.

Document migration for img-responsive to img-fluid

Remove dropped form-horizontal from forms.md

Add .navbar-toggleable-md class

In Bootstrap 3, the menu collapsed on the -sm- breakpoint, I believe the equivalent of this breakpoint in Bootstrap 4 is actually the -md- breakpoint. The navbar currently has no option to collapse at the -md- breakpoint and I'd like to add this.

Add a license header to bootstrap.scss

v4 - Rename .img-responsive class to .img-fluid

v4 - Fix Navbar at top of v4 Offcanvas example

Fix missing anchors and 404 errors.

Replace instances of 'control-label' with 'form-control-label'

[skip sauce]

clarifies that normalize.css is a dependency of reboot

Fix #17173

Fix #17166

Media vertical align flexbox classes

v4 - Force linebreaks after grid tier names in v4 grid docs

v4 - Fix link to narrow jumbotron example

v4 - Updated default navbar example to use responsive navbar