feat(task): notify on new task

[jokruger]

Use onesignal (if configured) to notify volunteers on new task submitted.

[teleyinex]

Can you also open a PR to the docs so we can have this feature in the documentation?

[teleyinex]

@jokruger the tests are passing, so we can merge but we would need two extra items to merge this new feature into PYBOSSA:

• Docs, just adding a PR to this file. Add a new sub-section with the new setting, and explain what it does.
• Tests for this new feature. As you are using Onesignal, just test that your line 149: requests.post(url="https://onesignal.com/api/v1/notifications", headers=headers, json=body) is called. Mock it, and we will be sure that it's being called when you enable it.

As I've integrated this branch, you can just push the code directly to it, and then we will be able to merge it. Thanks a lot!!! And if you need help, let me know it.

[teleyinex]

Reopen it when you have integrated the changes that we requested.