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 
The code is actually correct and my following suggestion is 100% subjective.  Subscripting can help improve readability a lot.  For example:

```objective-c
suggestion.userLogin = [dictionary stringForKey:@"user_login"];
suggestion.displayName = [dictionary stringForKey:@"display_name"];
suggestion.imageURL = [NSURL URLWithString:[dictionary stringForKey:@"image_URL"]];
```

Would become:

```objective-c
suggestion.userLogin = dictionary[@"user_login"];
suggestion.displayName = dictionary[@"display_name"];
suggestion.imageURL = [NSURL URLWithString:dictionary[@"image_URL"]];
```