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 
My bad @allendav.  I think it's best to keep this code as you had it initially, since `[dictionary stringForKey:@"key"]` is not really equivalent to `dictionary[@"key"]`.

Your initial code was more robust and correct.