Add API endpoint to create private 1-on-1 conversations
Add API endpoint to create private 1-on-1 conversations

A separate endpoint at POST /user/id/chat to get or create a 1-on-1 chat
between the logged in and the given user.

yunity/foodsaving-backend,yunity/yunity-core,yunity/yunity-core,yunity/foodsaving-backend,yunity/foodsaving-backend