Issue/2535 add mentions comment notification replies

This pull request adds REST API powered contextual @mentions suggestions to the reply text view when viewing comment notifications for WPCOM blogs.  Props @oguzkocer @drw158 for their reviews thus far.  cc @sendhil 
Actually, if `dictionary` is coming from the REST API, and `dictionary[@"image_URL"]` is documented to be a string there's no need for `stringForKey:`, so it's OK as it is.