[WIP] Host evidence uploaded to openbadger via "issue with evidence" 

This PR makes openbadger hold on to the evidence uploaded via the "Issue One With Evidence" functionality, rather than destroying the evidence once the badge is claimed.  It also adds an evidence display page which can be found at /badge/evidence/<assertionhash>

Note that there isn't currently any natural way to navigate to this new evidence display page, nor is the page itself particularly attractive.  I was worried about displaying any user information on the page, so it doesn't have any indication as to whose badge you are looking at, which makes it feel a little weird.  Currently this page requires no authentication, simply because I wasn't sure how private evidence is supposed to be.  Should it be admin-only?

I welcome any comments, as I'm not especially happy with this implementation, and regard it as a very rough first pass.