Update docs to include mention of the custom variables file

Merge branch 'provide-custom-scss' of https://github.com/stephaniepurvis/bootstrap into stephaniepurvis-provide-custom-scss

Prevent breadcrumb separator from getting underlined on hover when not using <ol> markup

Use proper variable for .dropdown-backdrop's z-index

Remove near-duplicate contextual list group example

Add variables for miscellaneous box-shadows

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Add variables for padding of popover title & content

Add variables for media object margins and padding

Docs: Use mixin syntax in Sass example of .invisible

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$enable-gradients docs: Add missing word

Fix RVM under Travis caching by excluding node-which from $PATH

See https://github.com/travis-ci/travis-ci/issues/5092 for more details.
Reverts #18376

Docs typo: from controls --> form controls

Allow optional breakpoint arguments to be passed on to nested media query mixins

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Use proper units when adding/subtracting lengths

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Rename padding local vars per new naming convention
/fyi @mdo

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Add Wall of Browser Bugs entry for #18859

See https://bugs.webkit.org/show_bug.cgi?id=153056
Refs #18859
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