Update Jekyll to v2.4.0 and rouge to v1.7.2.
Add labels explanation to readme
Update grid-framework.less

The `when (@index = 1)` guards are not needed cause the mixins already only match on arity.
Use RegExp for button plugin's focus shim

Fixes #14923.
Merge pull request #14925 from twbs/button-focus-use-regexp

Use RegExp for button plugin's focus shim
grunt
Examples: Make use of the `footer` tag instead of a `div`.
Use jsDelivr CDN until BootstrapCDN gets updated
Merge pull request #14930 from twbs/use-jsdelivr-cdn-urls

Use jsDelivr CDN until BootstrapCDN gets updated
Fix the iOS 8 unit test failures

Resolves #14919.
Merge pull request #14931 from twbs/fix-ios-unit-test-failures

Fix the iOS 8 unit test failures
fix typo in comment
Fix #14936: Use correct left/right and margin on tooltip arrows
Replace v3.2.1 reference with v3.3.0 since there wasn't a v3.2.1

Closes #14941
Replace deprecated uses of .pull-left & .pull-right in .media docs

Refs https://github.com/twbs/bootlint/pull/153
[skip sauce]
Explain issue/PR bots in CONTRIBUTING.md

[skip sauce]
[skip validator]
Merge pull request #14945 from twbs/media-pulls-docs

Replace deprecated uses of .pull-left & .pull-right in .media docs
Revert "Use jsDelivr CDN until BootstrapCDN gets updated"

This reverts commit ea23969150e7329dd4e996b844db0282c4d2319d.

BootstrapCDN has now uploaded Bootstrap v3.3.0
grunt
Customizer: listen to the right event for manual file selection in import