inv run should start both flask and celery

[igor47]

currently, most people run the site locally with inv run which starts flask in debug mode. however, if you send emails or do other delayed tasks, those do not happen. you have to separately run inv run.celery-worker to get celery running.

it would be nice if inv run ran both components. alas, there is no good way to use invoke's run function async (see e.g. pyinvoke/invoke#631). in particular, there's no way to terminate those tasks except by sending them a ctrl-c. might be some hackery possible with threads or something....