Reactions: Allow all emojis

Problem: At the moment you can only react using a small set of allowed emojis. The upside of this is a simpler UX, however it limits creativity and engagement. It also prevents more flexible uses of reactions. E.g. Polls.

Can we use our existing emoji picker to let people react with any emoji?

👽💩🫠💅🦄

NB. One potential issue is that at the moment we have plain text descriptions of our reactions - these appear when you hover over a reaction. Is this possible if we allow every emoji?