[ci skip]

Introduce $btn-block-spacing-y variable

Uncomment Firefox <progress> styles
/fyi @mdo

[skip sauce]
[skip validator]

Refactor Breadcrumbs component

Use new `.breadcrumb-item` class instead of child selectors and `li` tag selectors,
so as to no longer require the usage of `<ol>`-based markup.

Rename variables to follow naming conventions:
* $breadcrumb-padding-vertical => $breadcrumb-padding-y
* $breadcrumb-padding-horizontal => $breadcrumb-padding-x

Introduce new variable: $breadcrumb-item-padding

[skip sauce]

Update shrinkwrap

Reorganize utilities into more separate files under a `utilities` directory

[skip sauce]
[skip validator]

Remove useless .col-*-offset-12 and .col-xs-offset-0 classes

Apply clearfix to .card-{header,footer,block}
/fyi @mdo

[skip sauce]
[skip validator]

Docs: small tweaks to utilities

as on first skimming over these, it may not be completely obvious

...otherwise, by the same logic, it should be "Bolded text" above. just
looks clunky...

Fix bulleted lists on Utilities doc

[ci skip]

Migration guide: .panel-title's closest replacement is .card-header

[skip sauce]

Use variables for .progress base colors
/fyi @mdo

Refs #18535
[skip sauce]

Refs #18535

Address IE10 in browser compatibility docs

[skip sauce]

Add RubyGems section to "Download" docs

Closes #17291 by finishing the last remaining task.
Also reorders the sections in the "Download" docs.

[skip sauce]

Introduce $nav-item-margin variable
/fyi @mdo

Introduce $card-deck-margin variable
/fyi @mdo; happy to tweak the var names if desired

[ci skip]

[ci skip]

🏃🏃🏃I don't wanna get diabetes. 🏥
Obey The Mickens!: https://vimeo.com/111122950#t=14m9s

[ci skip]

remove generated content value for progress