Hide Settings Sidebar Based on Role

Very similar to https://github.com/TryGhost/Ghost/pull/3356 - the Settings sidebar links are now restricted based on role.
Currently there is a limit of one Role per User. The codebase will support more in the future -- when that time comes, there should probably be a client side permission system that would replace this code. There is further discussion here, on an earlier PR: https://github.com/TryGhost/Ghost/pull/3356