Detect if term-specific legislative position being used and prevent creation of superclass positions

[gbp]

From: everypolitician/membership-comparison#25 (comment)

So where we find a term-specific legislative position being used, I suspect we might want to not only raise a clash for this suggestion, but to prevent the creation of any P39s to the superclass (to prevent the case we actually hit with India, where we were unaware that the people actively working in that area had already decided to use the term-specific items). This might be one we want to find a way to detect "in the large" at either page creation time, or when loading all the statements, rather than only on a suggestion-by-suggestion basis though, as it largely means the entire Verification Page is probably invalid, and should be re-pointed at a different membership item instead.