Chevron for panel state

Adds chevron to panel title, showing their collapsed/expanded state and allowing another clickable thing (next to title) to expand/collapse panels. Used on "Admin Config" for third party OAuth settings.

Based on http://stackoverflow.com/questions/18325779/bootstrap-3-collapse-show-state-with-chevron-icon and http://jsfiddle.net/panchroma/3gYa3/ (mentioned in answers); note that I removed the `color: grey;` here, to allow a theme to control that. Perhaps there is a better `@` variable that could be used instead (dimmed text or so)?