cover more sensible defaults

src/store/initial.js

@@ -91,7 +91,6 @@ const initial = {
     },
     cover: {
       title: 'project title',
-      subtitle: 'project subtitle',
       description: 'A description of the project goes here.\n\nThis description may contain markdown.\n\n# This is a large title, for example.\n\n## Whereas this is a slightly smaller title.\n\nCheck out docs/custom-covers.md in the [Timemap GitHub repo](https://github.com/forensic-architecture/timemap) for more information around how to specify custom covers.'
     },
     loading: false