Bump SCSS-Lint, and `bundle update`

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Add Wall of Browser Bugs entry for #16915

https://connect.microsoft.com/IE/feedback/details/2274240/edge-edge-renders-an-svg-img-with-max-width-100-too-small
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Add prefix to docs/index.html assets/img/sass.png

Replace inline HTML with markdown, rewording, and cleanup.
Closes #18984

Refs #18984
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Document .small and .mark classes

These are available in _type.scss but not documented.  These classes are useful for styling without adding semantics.

Add Wall of Browser Bugs entry for #18228

See https://connect.microsoft.com/IE/feedback/details/2263132/edge-ie11-subpixel-problem-with-transparent-borders
Refs #18228
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Fix typo in docs/assets/scss/docs.scss
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See http://openradar.appspot.com/24235301
Refs #17497
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Add Wall of Browser Bugs entry for #17497

https://bugs.webkit.org/show_bug.cgi?id=153224
Closes #17497

Add pointer-events:none to custom indicator

Closes #18887

Add Wall of Browser Bugs entry for #18543

https://connect.microsoft.com/IE/feedback/details/2256049/edge-ie11-viewport-rule-has-side-effect-of-making-scrollbars-auto-hide
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Fix active Home tab in narrow-jumbotron example

This reverts commit e7b0d726.
Closes #17022
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Rename gradient mixins to match new naming convention

Respect $border-rounded inside progress bars too

rm unnecessary border-radius properties from .btn-group-vertical code

v4: Custom forms variables

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