Emoji Reactions

This makes it possible to 'react' to a post with a single character unicode emoji. This adds a new object type called EmojiReaction. It looks pretty much like a Like, except it also has a content property.

I decided against overloading Likes because they really are semantically different. Just as one aspect, there are a lot of 'negative' Emoji that should not be treated as a like.

A frontend could still decide to display the reaction and like count in a combined way and only differentiate on expanding a post.

Mastodon-API style frontend endpoints have been added for reacting and getting those reactions.