Allow user to choose any existing emoji icon for any reaction spot#11

Right now as you build a custom Reaction Set, you only can choose from a subset of the icon library for each spot (Like, Joy, Neutral, etc.). That limits the ability to create a completely custom set of 5 that can collect the emotional reactions that matter most to your use case.

We can already change the name of the reaction for each spot. We should be able to choose any icon from the library for any spot, not just a narrowed down group.