Fix an information disclosure when requesting access to a group containing private projects

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/19102.

The commit speaks for itself:

    Fix an information disclosure when requesting access to a group containing private projects
    
    The issue was with the `User#groups` and `User#projects` associations
    which goes through the `User#group_members` and `User#project_members`.
    
    Initially I chose to use a secure approach by storing the requester's
    user ID in `Member#created_by_id` instead of `Member#user_id` because I
    was aware that there was a security risk since I didn't know the
    codebase well enough.
    
    Then during the review, we decided to change that and directly store the
    requester's user ID into `Member#user_id` (for the sake of simplifying
    the code I believe), meaning that every `group_members` / `project_members`
    association would include the requesters by default...
    
    My bad for not checking that all the `group_members` / `project_members`
    associations and the ones that go through them (e.g. `Group#users` and
    `Project#users`) were made safe with the `where(requested_at: nil)` /
    `where(members: { requested_at: nil })` scopes.
    
    Now they are all secure.

See merge request !1973