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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Merge branch '3.0.0-wip' into bs3_drop_accordion_for_panel

Conflicts:
	dist/css/bootstrap.min.css
Merge pull request #9404 from twbs/bs3_drop_accordion_for_panel

BS3: Drop the accordion for the panel (fixes #8648)