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 
"Typedef"ing the block type instead of writing it inline can be a good thing for the caller (in case the block is not written inline on the method call) and make the code easier to read.

Example typedef:

``` objective-c
typedef void(^SuggestionAvatarFetchSuccessBlock)(UIImage* image);
```

That would make the method become:

``` objective-c
- (void)fetchAvatarWithSize:(CGSize)size success:(SuggestionAvatarFetchSuccessBlock)success;
````

And if the caller doesn't want to define the block inline, it can become:

``` objective-c
SuggestionAvatarFetchSuccessBlock successBlock = ^(UIImage* image) {
     // Do something...
};

[suggestion fetchAvatarWithSize:size success:successBlock];
```