Ensure that group owner cannot request access to a project of their group

## What does this MR do?

It fixes two things:

- 91ad995d69e1a0f8991fd896f1d9febc109273fe Ensure that group owner cannot request access to a project of their group
- ec3ff061148d556757e7cd486cdc6083d77acf34 Ensure group/project owners can see their members' access_level (see the commit message for details)

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

Not really, these are pretty simple fixes.

## Why was this MR needed?

Because there was an issue created!

## What are the relevant issue numbers?

Fixes #18717.

## Does this MR meet the acceptance criteria?

- [x] CHANGELOG is not needed since the bug is only present in a 8.9 RC
- [x] Tests
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4729