Add a blogger role to Debut Blog

Problem/Motivation

A fairly common use case is that individuals in an organization or community may be granted access to maintain a personal blog without having broader content access permissions. For this use case, the standard Debut "contributor" role is inappropriate, as it includes create access to other Debut-provided content types.

Proposed resolution

Provide a "blogger" role.

Remaining tasks

The current Debut feature spec specifies that Debut features may not provide roles beyond the ones defined in the spec. Would need to relax this requirement in the spec.