remove app.yaml description and use the readme instead

[lucarin91]

We decided some time ago to use only the README.md for adding additional description to an app or an example instead of the description field in the app.yaml, but it was never implemented.

To make that work, we should:

• parse the README.md
• get the first paragraph (so no titles)
• remove all markdown syntax
• return that in the app description field in the api endpoint for the app

After that, we should also remove the description field from the template app.

[lucarin91]

connected with arduino/app-bricks-examples#31