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Remove `gem install scss_lint` from Travis.

Clean up inclusion of IE-specific JS files.

Move them to footer, and remove duplicate ones.

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Instead, use `bundleExec: true` for grunt-scss-lint.

Port skiplink styling from v3 to v4

Compress images.

Remove all uses of `selector &` to support nested import

add Chinese translations website boot4.com

Update dependecies.

Update Holder.js to v2.8.2.

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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