We're going fancy.

Because sending emails can take some time, especially if we set the
reviewer of 100 series from the web page, the mailing is done in celery
tasks.

I chose redis as the message broker as it can be used for other things
as well, like a memcache-like cache if we ever want to go that route. It
can also be used for communication with a daemon that would support long
lived connections (think web socket or server sent events)

Also, to try and be a bit generic, emails are rendered from templates.

Unit tests cover everything I could think about, including not notifying
users that have set the reviewer to themselves.

Fixes: https://github.com/dlespiau/patchwork/issues/28


Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>