Add configurable welcome page, move content of home to it

[jryans]

• Riot will have two configurable .html pages:
  • welcome: the page you'll see set against the photo background, outside the app, advertising things which are on the inside
  • home: the page you'll see within the app.
• Both are optional
  • If you configure a home, you'll also get a home button in the top left menu
• You're never expected to go back 'out' to the welcome only to come back 'in'
• Default Riot on https://riot.im/app won't actually have a home page

[jryans]

Fixed by #8466 and matrix-org/matrix-react-sdk#2597.