Tasks scheduler using huey

[borja]

Follow up of #29 (signup users).

We have the code that run a separate process in a task to handle emails queued in the mail queue.

That task code can be run from the command line, within the repo, like this:

  python ow/tasks/run.py development.ini send_emails

And that can be run periodically using a cron job, but that is not optimal for self-installations (like someone running openworkouts for personal purposes in a laptop - well, should someone in that setup needs email verification anyway??).

Using a tasks manager would be a good built-in solution with zero setup and huey (​https://huey.readthedocs.io/en/latest/getting-started.html) sounds like a good choice (lightweight and easy to implement)

So, look into how to hook our tasks into it and run them through it.