Add errorBuilder route to NotFoundScreen

[bizz84]

Related: #148.

This fixes the issue by showing a NotFoundScreen when navigating programmatically to a route that doesn't exist.

Also added a rewrites section to firebase.json:

{
  "hosting": {
    "public": "build/web",
    "ignore": [
      "firebase.json",
      "**/.*",
      "**/node_modules/**"
    ],
    "rewrites": [
      {
        "source": "**",
        "destination": "/index.html"
      }
    ]    
  }
}