Currently we effectively create the reactions list in EventHandler then pass that data into a model. This reworks the model so that we just pass in a room and an event and it grabs it's own data. This means that:
- the functions in event handler are no longer required
- the model can update itself to add/remove reactions so no need to handle that in MessageEventModel
- MessageEventModel only needs to create new ReactionModels or remove old ones when no reactions exist anymore
A basic test suite has also been created for the ReactionModel
Create a reaction model that provides all the required data for `ReactionDelegate` so that none need to be calculated in QML.
This also cleans up the API for `ReactionDelegate`
