Retention rules - Expose flexible retention during creation

As a records manager, I can create a rule that leverages flexible retention

AC
When creating a retention rule
A new option is available as a checkbox in the form

• Label: Allow undeclaring the record
• Default value: false
  Option is located below the "rule is active" option as a dedicated line

When the form is saved, the option sets the value of the retention_rule:flexibleRecords property for the document

• true if checkbox is checked
• false if checkbox is unchecked