Add repositories

- Make model classes imutable
- Move model classes to common module
- Add repositories for backend and frontend
  - Backend repositories communicates with sqlite
  - Frontend repositories commuincates with rest api
- Add rest api
  - Authentication via cookies
- Add coroutines to forntend
- Add change listener to repositories
- Transmit change events to frontend repositories via websocket
- Switch to server side sessions
- Move setup from ui to cli

.gitignore

@@ -2,6 +2,7 @@
 .idea/
 build/
 web/
+.sessions/
 
 *.swp
 *.swo