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 
Eventually we may want to expire them after a set amount of time, but for the purpose of this initial commit of the feature @oguzkocer and I were satisfied that the cache clearing as the app is relaunched is sufficient - especially since the suggestions right now are only based on the users of the blog.  When we add suggestions based on commentors in the thread we'll be coming back to this everywhere mentions are used (not just the iOS app)