Apply permissions to MPCommentAnnotation objects

Needed soon, to enforce the restriction that a comment has an “owner” and only they are allowed to update it:

• The contributions property of MPCommentAnnotation must be immutable, it must only contain one item, and the profileID of the MPContribution object in the contributions array must match the current user's _id (i.e. they're the owner of the MPCommentAnnotation).
• Only the owner of the MPCommentAnnotation object is allowed to update it.

Needed but not quite yet:

• A project "viewer" should be allowed to create MPCommentAnnotation objects in that project.