Refine homepage UI

• Add a subtitle to explain what the app does and how to use it, and introduce the buttons.
• Shorten the button text accordingly.
• Use ellipses where needed.
• Give one of the actions a better name when sandboxed.
• Move that action to the right since it's probably the least likely
• thing to be clicked on, so demote it in visual importance to match this.

Before	After

@teams/vdg @sitter