Update conversations according to the design

[danpecher]

• Added extraData to Conversation model in Firestore module
• Passing the fonts from the parent app to the ChatUI module is done using FontConfig struct. I made the chatUIFontConfig global variable so the view classes can easily access it and it doesn't need to be passed down through many classes. I'm open to better solutions though.
• I kept the color definitions inside ChatUI module but it could also be part of some config struct passed from the parent app
• I used ViewModelingState from STRV template
• I updated the file structure of ChatUI module a little
• For now there's no image caching so the avatar image is downloaded again on every cell dequeue. The question is if we should create some custom simple solution, use a pod or offload the caching somehow to the parent app.
• I also added some extensions for convenience.

I had to make some changes to the Chat module as well:

• Add fonts
• add "compatibility" field to the conversion from network model to UI model