Admin award already awarded achievements

PR is WIP, and should not be merged yet.

Related to #100. For badges that have been already awarded to a user only a revoke link appears. This even is the case when a badge can be earned more than one time by users.

Issue: If the link is always visible and is clicked, a badge that can only be earned 1 time can be earned more than 1 time by the admin awarding it. Should these be checked?

Proposal: We check if a badge is valid to award to the user, and only display the award link in those cases. Possible issue though if that check is expensive and a site has a lot of badges to display in the table.