Replace placeholder icons and screenshots with real ones

[brettdh]

Motivation

In order to get the app into iOS/Android beta testing, I've put in some icons and screenshots that I grabbed as quickly as I could, just to check the box and have something there. These should be replaced with the ones we want to show when we hit the App Store and Play Store.

Plan of action

@tannerhearne - Either you can do this, or you can describe what you think the screenshots should be, and I can capture them. If they involve annotation like I've seen in a lot of apps lately, you should probably do them. For the icon, you may want to experiment yourself to find something that looks right for each store - I noticed there were several different sizes and paddings in the various brand assets in Google Drive, so I leave that to you as well.