Fix project member access for group links

What does this MR do?

Among other things, ensure that users who have access to a project through a group link can see confidential issues.

Are there points in the code the reviewer needs to double check?

I tried to keep the change as minimal as possible.

Why was this MR needed?

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.

Does this MR meet the acceptance criteria?

• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #23872 (closed).