[dash] "Get started > Test drive": copyedits and figure fix

[chalin]

• Move start-app images into assets folder
• Move _app-figure.md into _includes so it can be shared
• Regenerated starter-app image (the previous one was out-of-date)
• "Get started > Test drive"
  • Use a <figure> with a caption instead of an <img>
  • Put a (light gray) border around the image
• Copyedits to "Get started" pages

Staged at https://ng2-io.firebaseapp.com/get-started/test-drive

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Screenshots

Image formatting was:

It is now:

[Sfshaza]

That's better. Thanks! LGTM