Previously, the valid_owner check used the Repositories List Teams API
endpoint to check if a team was valid. Due to an issue in the
GitHub API, that endpoint does not return child teams that would
inherit a parent team's permissions.

This commit changes the initial check for the validity of a team to use
the Teams List Teams API endpoint, which lists all teams in a org to
check if the team exists at all. If that check fails, it will clearly
state that the team does not exist.

As a follow up to the initial valid team check, this commit checks the
Teams IsTeamRepoBySlug endpoint, which returns an object containing the
actual permissions for the team being checked on the repo. By adding
this check, we can verify that a user can actually provide their review
on PRs that made the CODEOWNERS line.