add `content_delete_role` to complement `content_delete_user` and `content_delete_group`

[kmasiello]

The deletion of permissions from content can come from a need to either:

• remove specific users
• remove specific groups
• bulk remove a suite of permissions from content, such as all viewers, or all viewers+collaborators

The first two are covered by existing functions. content_delete_role(c("viewer", "collaborator", "all")) addresses the latter case

[toph-allen]

This issue was referred to by the connect-cookbook file revoking all access except the content owner. Mentioning that here so we can remove that TODO comment.