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 
If there's an error, it seems like we won't be able to update suggestions for this siteID anymore until the app is closed.

What I mean is... shouldn't we add the following line to the failure block as well?

```objective-c
[weakSelf.siteIDsCurrentlyBeingRequested removeObject:siteID];
```