Test different backgrounds for DMG install
At the UX sprint, one of the users didn't seem to understand that the arrow in our DMG background meant to drag the app into /Applications and then hit legacy/trac#14630 (moved). Mrphs pointed out that we might A/B test some versions of the background that were more suggestive, such as the one that Adium uses: https://trac.adium.im/wiki/HowToUseDiskImages