Use one mail transporter for all sites

[nlsvgtr]

The mail routine needs a site config to determine which transporter to use. Since it seems highly unlikely that different mail transports would be used for different sites this should be changed. One mail transporter for the whole application should suffice.

When this is done it would be possible to merge messages to the same email address into one email, even when hey target different sites, in e.g. src/cron/send_site_issues_notifications.js