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 
Again, aiming for standards this would be:

```objective-c
- (void)suggestionsTableView:(SuggestionsTableView*)tableView didTypeInWord:(NSString *)word;
```

Even if the table view is not directly used, it makes the method immediately recognizable as a delegate method, and makes a name clash very unlikely to happen.