Add weekly reminder cron task

[mapiolca]

Summary

• declare a scheduled weekly reminder job in the module descriptor
• implement the TimesheetweekReminder cron handler to validate settings, select recipients, and send reminder emails
• add translation strings for the reminder cron job and runtime errors

Testing

• php -l class/timesheetweek_reminder.class.php
• php -l core/modules/modTimesheetWeek.class.php

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Codex Task