Stopgap re deletion of donation sites

[cielf]

Partial (just a stop gap) re #4204

Description

This PR prevents the deletion of donation sites if they have donations.

Note that this is only preventing the deletion behind the scenes-- it is strictly a stopgap that we can put in "immediately" if the remainder of the work for #4204 is not yet ready. (I think the remainder of 4204 will be independent of this)

It is just restricting the deletion of the donation site to cases where there are no donations. It leaves the button in place. This is not by any means perfect, but will prevent the most egregious results.

I used restrict_with_error. I considered restrict_with_exception, which would "blow up" iin the bank user's face so they would know something was not right -- whereas this is going to die silently. I chose restrict_with_error because I didn't find any cases of restrict_with_exception in the codebase on search.

This is meant strictly to be a band-aid. Hopefully I will get at least the next step completed for this week as well.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

[dorner]

Looks good to me!