Start building mobile discussion page

Everything has been developed on the "next" subdomain. So if you
go to next.loomio.dev (if you're using pow) and visit a
discussion page, you'll see the new layout.

The new page is using bootstrap 3, so we've had to use another
bootstrap gem so that we don't compete with the bootstrap 2
installation currently in the app.

All of the stylesheets, views, and assets are namespaced in "next"
directories so development can be completely separate from the
main app.

Also, the stylesheets have all been implemented using an atomic
file-structure and BEM naming-conventions.