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.  Fixed in https://github.com/wordpress-mobile/WordPress-iOS/commit/ca8f87ea835a3e25e2e89ca4e6a18b2804f06d7c