only allow Concept Difficulty editor for existing levels
only allow Concept Difficulty editor for existing levels

rvarshney/code-dot-org,pickettd/code-dot-org,rvarshney/code-dot-org,pickettd/code-dot-org,rvarshney/code-dot-org,rvarshney/code-dot-org,rvarshney/code-dot-org,pickettd/code-dot-org,rvarshney/code-dot-org,pickettd/code-dot-org,pickettd/code-dot-org,pickettd/code-dot-org,rvarshney/code-dot-org,pickettd/code-dot-org,pickettd/code-dot-org,rvarshney/code-dot-org