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 
Breaking this method up would help improve readability + maintainability a lot IMHO.  For instance, all lines related to the tableView initialization could be replaced by something along the lines of:

```objective-c
[self setupTableView]
```