Improve authentication story for edit sessions

[joyceerhl]

@jrieken had the following feedback while setting up for testing #158798:

1. There is no Sign In entry or badge in the accounts menu badge, only a command palette option to Sign In
2. Expected to see edit sessions as an option when enabling settings sync. More broadly, expected a unified 'Sign In to VS Code' experience, which is also feedback that @Tyriar had
3. The 'Signed In' group for existing auth sessions is confusing--it means that there's an existing auth session for that auth provider in general, but can look like the user already used that auth provider to sign in for edit sessions (@lramos15's feedback in Create New Codespace fails if the current repo was cloned using SSH #159287)

[joyceerhl]

Verification:

1. First turn off edit sessions from the account menu if you had it turned on before
2. Make some changes in insiders.vscode.dev/github/microsoft/vscode
3. Run the Continue Working On command
4. Verify you are asked whether you want to bring your working changes with you using edit sessions
5. Pick an account to use with edit sessions
6. Run the Continue Working On command again
7. Verify that you aren't asked again about bringing your working changes with you