Project members: Cleanup Markup and Code

In https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28436 we introduced a couple of template changes and mobile fixes for the Group member overview.

The Project member overview uses a very similar markup. The mobile fixes introduced in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/28436 should be applied to the Project member template as well.

With this, a lot of page specific styles in member.scss might become obsolete and should ideally be removed.