Staff cannot record a confirmation

Problem/Motivation

If a confirmation is required, but for some reason we can't send an email then staff can send a confirmation manually - e.g. post a letter. However once they have saved the original declaration, it becomes locked immediately and they no longer have permission to edit the confirmation_date field.

Proposed resolution

1. Remove static from RecordBase::getCriticalFields() (we don't use it statically anyway).
2. Change Declaration::getCriticalFields() so that it adds 'confirmation_date' only if isConfirmationSent()
3. Optional: and adds 'validity' only if isClaimable() is TRUE.

Item 3 is for a different situation, related to #3573666: Flexibile cancellation - it allows staff to invalidate a single declaration without forcing to proactively cancel.

Remaining tasks

User interface changes

API changes

Data model changes