Don't show "request access" button to project owners
This MR fixes an issue where project owners that are not in the project's members list (I believe this is how we handled project owners before, now we seem to create a "Master" member for the project creator) would see the `Request Access` button.
This MR fixes this issue in a clean way by adding a new `:request_access` ability to replace an ugly helper.
It also give project owners the ability to update & destroy a requester that would happen to be themselves (since owners could request access to their own project before this MR).
Related to #18790.
See merge request !5091
8a62c12d38