Store conversations locally in db

[FelberMartin]

Description

While working on #375, I noticed that conversations are not stored locally in the db. This leads to problems, as we listen for websocket updates in the conversationOverviewModel, and update the unread counter when new post creations arrive. However these counter updates are only local to the conversationOverview and we can not consume them in the ConversationViewmodel. There we would need the updates to properly show the unread indicator.

Additionally, Julian discovered this bug that may also be mitigated by storing the conversations locally.

Next steps

• Create db entities for the different conversation types
• Create a ConversationRepository that handles the snychronization of the db
• Confirm that both mentioned bugs are resolved
• Remove the feature toggle for the unreadIndicator