`ProjectTeam#find_member` doesn't take group links into account. It was
used in two places:

1. An admin view - it can stay here.
2. `ProjectTeam#member?`, which is often used to decide if a user has
   access to view something.

This second part broke confidential issues viewing. `IssuesFinder` ends
up delegating to `Project#authorized_for_user?`, which does consider
group links, so users with access to the project via a group link could
see confidential issues on the index page. However, `IssuesPolicy` used
`ProjectTeam#member?`, so the same user couldn't view the issue when
going to it directly.