Create initial UI designs

In light of https://github.com/dwyl/library/issues/8#issuecomment-336501544 we need to create some initial UI designs in order to start creating the actual app.

I tend towards everything looking a little material design-esque just because it's easy, but perhaps with @finnhodgkin's skilled eyes we might be able to do something a little more interesting.

I personally liked using [Gravit](https://www.designer.io/) but am open to other things.