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 
Agreed.  Nice catch!  Fixed in https://github.com/wordpress-mobile/WordPress-iOS/commit/9f79e3694e22ded1415b33aa8d3b053a88518bba