Add synchronisation mechanism

[tylp]

When the app initialize, a synchronisation must be done in order to update the remote instance or the local instance.

1. If the user wrote a note from another device, the current device must be updated with the latest changes.
2. If the user wrote a note while being offline, one the connection is active again, the local changes must be pushed
3. If there are local changes and remote changes, a merge must be done