UI improvements in project / environment permission configuration

Currently in the screen for managing user permissions for environments and projects, we show all users in the organisation, regardless of whether they have permissions to the environment / project. This can be confusing, and difficult to work with in organisations with larger numbers of users.

In order to improve this permanently, we would likely need some UX design work, but it would be good to at least add some clarity in the UI on what 'Regular user' means.

Please bear in mind that this may also be obsolete depending on the decision made in #5224 .