Identify instance with user id instead of username

[pixtron]

Describe the bug

Currently an instance is identified by server, username and context. If the username changes an existing instance would not be found anymore.

To Reproduce

1.) Login with user A
2.) Logout and login with user B
3.) Change username of user A
4.) Logout and login again with user A

Expected behavior

Existing instance for user A should be loaded.

Environment

• desktop client version: v1.2.1

Additional context

Add any other context about the problem here.

[pixtron]

For migration reasons, we would need to keep the existing check for a while, to handle cases where an instance does not yet have the userId set.