Carousel: Use the transition duration from CSS

This makes it possible to change the transition duration in CSS
enable CSS source maps in grunt-contrib-less
Add new form feedback states, clean up existing form styles

New stuff:

* Adds `.has-feedback` and `.form-control-feedback` to show icon-based
form feedback to users. Fixes #11836.
* Removes the `width: 1em` on `.glyphicon` because it makes it
impractical to resize the width of all icons, even if they’re not
“empty” (e.g., a simple `.glyphicon { width: 30px; }` wouldn’t work,
nor would a class preceeded by a parent class.

Clean up:

* Sets `width: auto;` on all `.form-control`s within inline forms.
* Removes all the inline `select` menu `width` styles because all
inputs are now `auto`.
* Update form states docs to separate out sections better.
Fixes #10492 again, this time for last row's corners on striped tables
copyediting
fix var name in example to follow JS conventions
fix typo in docs
simplify JS snippet slightly
use unique IDs for modal labels in doc examples so HTML validates
use unique IDs for input feedback examples so HTML validates
explicitly set more HTML5 validator options
Merge pull request #11886 from twbs/html-validator-config

explicitly set more HTML5 validator options
minor refactoring of 4ee93c9f8529cfeef56d4178b91e1ddcb0bff09c
make S3 caching significantly more generic
Merge pull request #11890 from twbs/generic-caching

make S3 caching significantly more generic
Merge pull request #11887 from twbs/minor-panels-refactor

minor refactoring of 4ee93c9f8529cfeef56d4178b91e1ddcb0bff09c
inform the people that fixed positioning ain't all it's cracked up to be /cc #11888
Merge branch 'master' of github.com:twbs/bootstrap
use S3-cached Jekyll install
Merge pull request #11891 from twbs/cache-jekyll-bundle

use S3-cached Jekyll install to further speed up the Travis build