First version of voila-viewer service

[elisabettai]

Service based on jupyter-math (voila) that waits for a file voila.ipynb in input_1 and renders it when it becomes available.

@odeimaiz, let me know if this will help you for kember-viewer and if you want some modifications before I publish the service

Known small issue With the following steps, a white page is rendered (instead of the custom one from flask or from voila):

1. Add the service
2. While it is pending or starting connect a voila.ipynb to input 1
3. As soon as the service is marked as "Ready", show its iframe
4. If 3 is done quick enough, a white page is rendered Reloading the iframe normally unblocks the situation

TODOs

• [ ] Find solution for the known issue
• [x] Improve the page that is displayed when the notebook is not provided in input_1
• [x] Add more information on how it works (e.g. an animation in this repository)
• [ ] Investigate why the container is marked as "UNHEALTHY" and provide a better health check.

Related issues https://github.com/ITISFoundation/osparc-issues/issues/694