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 
No worries.  Reverted in https://github.com/wordpress-mobile/WordPress-iOS/commit/3912ebe297fad177c28e4ed5d2340b0d2e48aa54