[BUG] Unable to add a strategy to property claims

[chrisdburr]

I've just noticed that a user is unable to add a Strategy element as a child of a property claim. Not sure if this has always been the case, and I've just noticed, or if something has changed.

Either way, it should be possible to add strategy elements to property claims (e.g. if a high-level claim, such as P1, needs to be broken down into multiple sub-claims by implementing a strategy for doing so.