[C2] Move AttendanceCalendar optional extra field "duration" to the c_attendance_calendar table

[ywarnier]

• create the c_attendance_calendar.duration field (decimal)
• update the entity
• add migration to create it, look for existing values in extra_field_values, transfer them to the new field, remove the extra_field_values, remove the extra_field
• remove the configuration setting (was as follows in C1)
• update the attendance calendar add/update form to add that this field is expressed in hours

This was the config setting in 1.11 (to be removed and the duration enabled by default in C2):

// Option to define duration for a calendar in attendance sheet.
// Create an attendance calendar extra field with field label "duration" of type "text".
// $_configuration['attendance_calendar_set_duration'] = false;

[ywarnier]

#6053 should already fix this