Before: 354 KB (363.514 bytes)
After:  129 KB (132.748 bytes)

tabs: fix nested tabs.

When one uses say a carousel inside a tab, the `.active` selector previously matches the carousel ones too leading to broken tabs.

It's not the perfect solution but should the job for now.

Also:

* compress them with SVGOMG
* add titles
* add `focusable="false"`

Feather link didn't go anywhere.

Changed feather link from `colebemis.com/feather` to `feathericons.com`

Fix HTML errors

Pretty useless, we need to switch to the official HTML validator ASAP.

I suggest that there's no longer a need to include the section on workaround for the IE10 bug on Windows Phone 8. Not only was this bug patched in a later release of Windows Phone 8 (which shipped in 2013), but Windows Phone 8's support ended a long time ago - even its successor Windows Phone 8.1 is no longer supported by Microsoft. The potential audience affected by this bug is likely microscopic so I suggest it can be safely removed. 

I suggest the workaround JS (http://getbootstrap.com/assets/js/ie10-viewport-bug-workaround.js) can also be safely removed from the docs and examples but I haven't done that in this PR.

* Fix to Chrome inheritance bug (#22872)

* Remove box-sizing from html

box-sizing removed from html and order of wild card selected and html selector swapped to maintain correctness of comments.

Custom File `$custom-file-focus-box-shadow` should show even if box shadows are disabled

Custom File variables should inherit from inputs

- Given buttons a required modifier (.btn alone is incomplete)
- Given the primary button, use a .badge-light for better contrast
- Consolidate example and highlight code with a `{% example %}` block

* add margin variable for breadcrumb

* add margin variable for breadcrumb correction

* breadcrumb margin variable changes after review

* reduce # of spacing rules by combining x & y with t r b l

* Remove unnecessary spaces