closes https://github.com/TryGhost/Team/issues/572
Role selection when inviting users was updated to show information about each staff role. This change updates the staff edit screen to use the same UI for changing the role of an existing user.
- added `select-user-role` modal which uses `<GhRoleSelection>` to allow role selection
- replaced the `<select>` input for changing a user's role with a button that triggers the role selection modal
- role is not set unless the "Set role" button is clicked after making a selection, cancel or close will leave the original role selected
