Don't display a Source link if no source is set
Don't display a Source link if no source is set

addresses #242

cityofsomerville/citydash,codeforboston/cornerwise,cityofsomerville/citydash,cityofsomerville/cornerwise,cityofsomerville/citydash,cityofsomerville/cornerwise,cityofsomerville/cornerwise,codeforboston/cornerwise,cityofsomerville/citydash,cityofsomerville/cornerwise,codeforboston/cornerwise,codeforboston/cornerwise