tests: run BrowserStack in parallel with Sauce Labs

Per https://github.com/twbs/bootstrap/issues/11087#issuecomment-31922999
and internal twbs team discussion.
Effectively reverts the deletions from #11701.
more detailed QUnit test results in Sauce
Merge pull request #12229 from twbs/richer-results

more detailed QUnit test results in Sauce
Merge pull request #12213 from twbs/browserstack

tests: run BrowserStack in parallel with Sauce Labs
revert unnecessary customizations of Blob.js & FileSaver.js

Per https://github.com/eligrey/Blob.js/pull/20
& https://github.com/eligrey/FileSaver.js/pull/63
The library author has deemed the existing /*! @source URL */ comments to be
sufficient attribution, and these already survive minification.

Refs ea591748edee4f8fe722274b71787ccf2b052c4a
upgrade tooltip/popover-in-groups callout from info to warning
downgrade <label>-related callouts from danger to warning
upgrade form-group + input-group callout from info to warning
downgrade .img-responsive cross-reference callout from warning to info
clarify docs regarding non-mixable components
Fixes #12210: badges in xs buttons
Merge branch 'master' of github.com:twbs/bootstrap

Conflicts:
	docs/assets/js/customize.min.js
Merge pull request #12231 from twbs/callouts

callout severity tweaks
Merge pull request #12232 from twbs/mixing-docs

clarify docs regarding non-mixable components