Add the ability to use sendmail rather than configuring a SMTP server

Hi,

In the case where the server on which Kresus is installed can send emails (typically if Kresus server and SMTP server are on the same machine), Kresus could use the mailing abilities of the server rather than requiring the user to configure a SMTP server.

It seems it is quite easy to do in nodemailer (see this doc). This should not replace the actual behavior, but be an alternative configuration to ease getting started with Kresus.

This is quite related to !434 (merged) and depends heavily on the future of the nodemailer dependency (see #612 (closed)). It might not be worth spending time developing this feature now, if nodemailer is to leave in the next release.