add role="alert" to Alert examples, to improve accessibility

Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/d8fcbcd3fe86f0cb47fda0ffe269b2c7a2131f07/README.md#alert
rework docs copy: inputs-without-labels are likely the more common case
Gruntfile.js: fix path after d08bbdc6ec50baca9e34d2aa6e8bcfc8d6135217.
Merge pull request #13024 from twbs/role-alert

add role="alert" to Alert examples, to improve accessibility
comply with .dropdown-menu.pull-right deprecation in docs
Update dependencies.
use stricter error checking in uncached-npm-install.sh

Closes #13045
Merge pull request #13039 from twbs/dropdown-pull-right-deprec

comply with .dropdown-menu.pull-right deprecation in docs
remove unnecessary direct devDependency on css-flip
upgrade to grunt-css-flip v0.2.0
Merge pull request #13048 from twbs/grunt-css-flip-0.2.0

upgrade to grunt-css-flip v0.2.0