wip:minor changes to charges

Waivers:

a) Should not be able to waive a fully paid charge
b) Waiver should be allowed only once per charge
c) Default behavior of a waiver should be to waive the total "unpaid" amount (not the entire fee)
d) Should not allow waiver of charges when a loan is not active (only active loans can be waived)

Delete:
a) Should not allow deleting a loancharge once a loan is approved (only SUBMITED_AND_PENDING_APPROVAL loans can have charges deleted)