Release v3.4.0

@mdo @Johann-S

TODO:

• Fix any 404s and useless redirects
• Remove customizer's gist creation
• Fix Browser tests
• Go back to single colon pseudo elements for backwards compatibility with the browsers we support in v3
• Fix docs search
• Replace ZeroClipboard with clipboards.js since Flash is dead
• Tweak clipboard.js styles
• Update team
• Nuke jobs from docs
• Add the unprefixed appearance property
• Add BrowserStack thanks in README.md
• Exclude docs less files from Jekyll
• Test builds with real apps just in case
• Verify sourcemaps work
• Customizer: remove drag and drop support and references, or fix this
• Make sure the SRI hashes are updated and correct!
• Make a dist commit on non-Windows and make sure there are no modifications; if there are, commit them, then rebase the branch
• Detailed references to the XSS fixes for each component in release notes/post
• Merge https://github.com/algolia/docsearch-configs/pull/577 after the release is live

Ship list:

• 29f9237f, 13bf8aea fixed XSS issue in Alert, Carousel, Collapse, Dropdown, Modal and Tab components, see https://snyk.io/vuln/npm:bootstrap:20160627.
• 48f14823 added a .row-no-gutters class.
• cf6f8671 Add padding to navbar-fixed-* on modal open
• 4f4592c2 and subsequent commits reorganized the v3 docs CSS to use Less.
• 1d832806 added a dropdown to the docs nav for newer and previous versions.
• 059969bc update the docs to use a new baseurl, /docs/3.4, for that versioning life when we deploy to GitHub Pages.
• 4ebe2b87 switch to BrowserStack for tests
• e105d156 use https/fix broken links in docs
• 25d017a2 replace ZeroClipboard with clipboard.js
• d7ab2fbd remove gist creation in customizer since anonymous gists are disabled by GitHub
• b304c613 remove drag and drop support from customizer since it didn't work anymore
• d24d47c2 Add docs searching via Algolia
• Remove the double border on <abbr> elements.

Closes #25679 (closed).

Docs Preview: https://twbs-340.netlify.com/