pe-accordions

Use ui.bootstrap.collapse in the sections menu to show/hide subsections. 

Had to do this as a directive to keep the collapse/expand var private.
We won't do that this way (see https://github.com/MoonshotProject/moondash/tree/ebr-gulp-angular-bootstrap), but let's merge it, and I will correct it in my ebr-gulp-angular-bootstrap branch.