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 
The more standard singleton method you'll find accross the WP app is `sharedInstance`.  This is small but it's the kind of stuff that can make searching for things easier. :mag: