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 
Small detail, but I think we could use some documentation explaining the meaning and proper use of these methods.  While this can be intuitive at times, I find that sometimes a small explanation from the developer can help newcomers a lot, and eventually also help maintainers avoid breaking stuff (due to misunderstandings).