Refactor all stateful widgets in the app to use cubits

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The list of cubits that we need to implement to port all functionality from stateful widget to (mostly) stateless widgets:

• NavigationCubit -- responsible for the navigation state (a single instance per app) (feat: introduce global navigation and routing #242)
• UserCubit -- currently logged in user (a single instance per app) (refactor: expose the user via a provider in the whole app #249)
• ConversationListCubit -- state and functionality of the list of conversations (feat: add conversation list cubit #267)
• ConversationDetailCubit -- state and functionality of the details screen of a single conversation (feat: add conversation detail cubit #264)
• MessageListCubit -- state and functionality of the message list (feat: add message list and message cubits #282)
• MessageCubit -- state and functionality of a single message (feat: add message list and message cubits #282)
• AddMembersCubit -- state and functionality for adding members to groups (feat: make add members screen stateless #284)