Feature/smtp user configuration

Implements user side smtp configuration to allow using custom or external mail service like requested by outdated MR !129 (closed) and !180 (closed)

Closes #192 (closed), #150

Configuration is made on /admin/install.php at first initialisation.

Default values are localhost:25 noAuth noTLS

Generated config.php sample:

// SMTP configuration
const SMTP_HOST = 'mail.moi.ovh';

const SMTP_PORT = '587';

const SMTP_TLS = 'on';

const SMTP_USERNAME = 'dev@moi.ovh';

const SMTP_PASSWORD = 'XXXXXXXXXXX';