cubicweb #3797393 Move config.sendmails to a service [open]

sendmails should be a service and not a function of config. It would make it easier to override it with a non-smtp system like mandrill for example.

The 'mandrill' cube (https://bitbucket.org/unlish/cubicweb-mandrill) provides such a service and monkey-patch cubicweb to use it. It can be used an example for this ticket.

It would be great to have this included in cw 3.20