fix adding/removing team reviewers with gh pr edit

[mercimat]

Fixes #3705

The issue:

According to the documentation of gh pr edit, the --add-reviewer and --remove-reviewer flags accept team users with the following format: myorg/team-slug.
If the PR doesn't have any team user specified in the "reviewers" section, the existing code works.
But if the PR already has a team user specified in the "reviewers" section, the gh pr edit 1 --add-reviewer myorg/other-team command fails with the error message: '' not found.

Identifying the root cause:

This error is returned by the TeamsToIDs function in api/queries_repo.go, because one element in names is an empty string.
Going further in debugging this, I found out that the empty string was generated by the Logins function in api/queries_pr.go.
The Logins function was using the RequestedReviewer.Login attribute for both normal users and team users. But teams don't have a Login field (see Team graphql reference). So, when parsing an existing PR with a team as reviewer, the Logins function returns a list with an empty string element.

Fix proposal:

This fix updates the graphql query to get the slug and the organization login in case of a team element, and it adds a check on the typename in Logins to build the login for a team concatenating the org login and the team slug. That way the team login can be compared to values in --add-reviewer, --remove-reviewer flags and in the metadata of the repo.

Testing:

I added a simple unit test for the Logins function. I also built the gh command with the changes and manually tried the gh pr edit command with different combinations of the --add-reviewer and --remove-reviewer flags.

Thank you in advance for your inputs on this PR. Please let me know in case changes to the fix would be needed.

[cliAutomation]

Hi! Thanks for the pull request. Please ensure that this change is linked to an issue by mentioning an issue number in the description of the pull request. If this pull request would close the issue, please put the word 'Fixes' before the issue number somewhere in the pull request body. If this is a tiny change like fixing a typo, feel free to ignore this message.

[samcoe]

This looks great! Thanks for the contribution. The thorough explanation of the details of the bug and the added tests made this very easy to follow 👍