Show users the number of products that match an expression

Issue
It's a recurring problem in our system that users configure (either intentionally or by mistake) an expression that excludes most (sometimes even all or almost all) products in the catalog. This causes performance issues, because the system needs to generate a huge number of recommendations to find the few that pass the expression.

Solution
We need to make this problem visible for Personalization Portal users. The idea is to add this information (the percentage of active products in the catalog excluded by the expression) to every place in PP where expressions can be defined (widgets, e-mail campaign, merchandising campaigns). Also, a threshold should be defined below which a warning will be displayed to the user. Let's assume 15% of matching active products as a starting point.