update download_dist for RC 2
update CDN links to point to continuously-updated build; addresses #9138 & its many dupes
don't show download_dist in favor of site.download; addresses #9138 & its many dupes
add HTML5 validation of docs pages to the build
add ARIA roles to Dropdown examples in Components docs
add src attribute to holder.js-powered <img>s so that they validate

use `data:` URL of empty invalid PNG: src="data:image/png;base64,"
add alt text to <img>s in components.html
add alt text to <img>s in javascript.html
Components docs: Dropdowns section: add .sr-only toggle buttons so aria-labelledby validates
Merge pull request #9396 from twbs/docs-html-validation

add HTML5 validation of docs pages to the build
Merge pull request #9389 from twbs/emphasize-git-HEAD

Emphasize git HEAD downloads
mention fubared state of overflow hidden on body in android and iOS to fix #8975 and #9315
Drop the accordion for the panel

* Deletes the accordion.less file
* Extends the panel to be collapsible with our JS plugin
* Supports panel color variations
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fix negative indenting in button group for nested button groups
fixes #8985: only specify minified dist files for bower
Added pagination-size mixin
Fixed errors
Fixes #9415
patch highlighting hint