[Finishes 67575866]: respond with 403 Forbidden instead of server erroring on CanCan exceptions
Merge branch 'master' of https://github.com/code-dot-org/dashboard into 403-instead-of-500-for-cancan-accessdenied
Merge pull request #340 from code-dot-org/403-instead-of-500-for-cancan-accessdenied

403 instead of 500 for cancan accessdenied