Admins can edit total amount due
 - feedback from conference admins is that frequently admins will need to change the total amount due for a registration for varying reasons.
 - caught a bug where total amount due would be wiped out by a subsequent update.
 - flow goes something like this now..
```java
if(updatedRegistration.completed() && !originalRegistration.completed()) {
	setCompletedTimestamp(currentTime);
}

//calculatedTotalDue derived from originalRegistration's completed timestamp and early registration factors
updatedRegistration.setTotalDue(calculatedTotalDue);

if(userIsAdmin) {
	updatedRegistration.setTotalDue(adminOverrideTotalDue);
}

registrationService.update(updatedRegistration);
```
I should check whether or not the logged in user has update rights on the conference, rather than admin rights on the registration.
 - Also, the logged in user should not be allowed to change their own amount due, for accountability
Fixing issues and comment from code review
Merge pull request #145 from CruGlobal/feat/edit-amount-due

Feat/edit amount due