Bootstrap3 upgrade

Hey @hypnotics, Rob and I had a couple hours on this today. we've got the navbar mostly working, and the group#show page about 95% there too. I'm not sure the best way to hand this off - do you want to read through our changes, and use that as a guide while you have a crack at the discussion#show page?

I'm going to be offline for the next couple days, but keen to support after that. Just play around, don't worry too much about breaking things, and let me know how you get on :)