Use VS Code authentication API to control GitLab accounts

Problem to solve

It's not obvious which GitLab accounts are stored in the extension. We also use custom way to log in and log out.

The only way to see the existing accounts currently is to run GitLab: Remove Account from VS Code command and then see the "accounts to remove" in the quick list:

Proposal

VS Code allows us to create many accounts. This wasn't the case just two years ago. When we do create multiple accounts, we'll have a nice overview called the "Account menu"

We'll use the VS Code authentication API to manage our accounts (they call it sessions)

For detailed investigation and screencasts of the new UI, see the spike result #1217 (comment 1902960104)

Further details

This issue is the last of the Single Account refactor and so we'll also clean up the old multi-account code.

Links / references