Get the app description from the readme if the field description is not present in app.yaml

[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

We also decided to keep the description field and use it instead of the readme in case it is present and not empty.

[lucarin91]

connected with arduino/app-bricks-examples#31, but this isn't required