create welcome screen / start dialog

[kfarr]

Add a new modal dialog in the Viewer that appears when no scene or street is passed in the URL hash

• This should be implemented in 3dstreet viewer, not the editor (css and source files should be separate). It should be hidden when an option is selected from the menu or when editor is invoked.
• Where does the list of streets pull from? This should be a json directory of scene json url, scene name, preview url
• What happens when user clicks various items on page?
• where does version number come from?
• are new svg / png needed? what path saved to?
• is there a way to access this screen again after start?

Design ready for implementation:

Hover and click states in figma: https://www.figma.com/proto/W9jid3A0jgssnIKBFEvG38/3DStreet-2022?type=design&node-id=4456-104880&scaling=min-zoom&page-id=2612%3A41679