Make invitation to click more obvious

[alice-sowerby]

In UX testing users were slow to see the invitation to click in the browser window to draw, and many sat and waited "for it to load".

It would be helpful for the invitation to be clearer.

[lukemarsden]

i.e. grey text less light grey

[lukemarsden]

or maybe the notice could be at the top of the screen, in some sort of header that makes it clear that the app is loaded and ready

[lukemarsden]

fixed in dotmesh-io@1d41a38