code cleanup: useless volatile keyword in scheduler stats variables
mail alerts: parametrizable subjects and html body with colors

3 alerts subjects depending of the severity of the most severe
information in the mail (new alert > reminder > cancellation):
* mail.alertsubject default: "NEW QRON ALERT"
* mail.remindersubject default: "QRON ALERT REMINDER"
* mail.cancelsubject default: "canceling qron alert"

Mail body is no longer limited to plain text but is now a MIME multipart
alternative with both text/plain and text/html bodies. The second one
uses colors to make alerts, reminders and cancellations more distinct.
This can be disabled by setting mail.enablehtml to "false".
ui: more consistent button colors

now light blue always means "link", and therefore dark blue now means
"download/data"
ui: fixed a minor display bug when custom actions were not set