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 
From what I'm seeing the suggestions are only downloaded once per site (unless the cache is cleared at some point).  Unless the app needs more memory, the objects will not be removed.

My question is: do we want these objects to expire after a set amount of time?