Temporary evidence should be kept when badge is claimed

[ghost]

Currently, openbadger relies on the client site to retrieve any temporary evidence attached to an issued badge and then pass it back to openbadger when claiming the badge. It should instead automatically attach the temporary evidence to the badge when it is claimed.

[cmcavoy]

Some bits that we'll need to do,

• create an evidence display page, similar to the criteria page
• link the evidence property of the BadgeAssertion.