Added the ability to reassign contributions when deleting donors

Treasurers can now reassign contributions to people or other donors when deleting donors. Previously you couldn't delete a donor record if they had any contributions assigned to them.

This is helpful in situations where someone made donations to your church prior to being a member, and then later became a member. This feature effectively allows you to merge a donor record with a person record.

You can also assign contributions to another donor when deleting a donor. This is useful when you have duplicate donor entries and want to merge the two donor records.

Note that any previously issued tax receipts for reassigned contributions will now be associated with the new person or donor, so never reassign contributions to an entirely different person or business. This feature is only intended for merging duplicate contributor records — for example merging a donor with the name "Jane Smith" with a person with the name "Jane Smith", or merging two donor records for the same company name.