correct enable selector for button usage

following the given examples the correct selector to enable the buttons with javascript is ".btn-group".
Address #9873 and fix #9774

* Update styling of justified nav links to be more mobile friendly
(with all four rounded corners and some margin between items)
* Add note to docs to callout bogus WebKit rendering on component and
example
Fixes #10137: correct classes on thumbnails
Add margin to thumbnails for proper spacing

Hadn't thought about this until fixes #10137, but it makes sense I
think to have margin on the bottom of thumbnails for spacing when
stacking (or using different grid tiers).
Improve upon #9937: Link and put reset column stuff to action in the grid example
Fixes #10016 and updates that rather badly formatted callout
Add missing period on customize page.
Merge pull request #10204 from premist/master

Add missing period on customize page.
[doc] tweaked the appearance of the last item of the file listings
Fixes #10207: alert-dismissable example requires contextual class
per the docs - use <a> instead of <button>
add  role="button"
Compress PNG files losslessly.

Use zopfli with 1000 iterations, pngout /r with various blocksizes and deflopt.

assets/ico/apple-touch-icon-114-precomposed.png | Bin 2939 -> 2931 bytes
assets/ico/apple-touch-icon-144-precomposed.png | Bin 3614 -> 3549 bytes
assets/ico/apple-touch-icon-57-precomposed.png  | Bin 1577 -> 1573 bytes
assets/ico/favicon.png                          | Bin 889 -> 887 bytes

Total difference: -79 bytes.
Update Respond.js to v1.3.0.
Update html5shiv.js to v3.6.2.
Add bugs property to package.json.
Merge pull request #10210 from acmetech/master

Update the example for components/#btn-groups-justified to match the "Element-specific usage" callout
Merge pull request #10209 from clbn/master

Fixes #10207: alert-dismissable example requires contextual class
Merge pull request #10208 from javiereguiluz/patch-2

[doc] tweaked the appearance of the last item of the file listings
Merge pull request #10215 from XhmikosR/master

Various