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, this full init method could be reduced to something along the lines of:

```objective-c
- (instancetype)initWithSiteID:(NSNumber*)siteID
{
    _siteID = siteID;
    _suggestions = [[SuggestionService shared] suggestionsForSiteID:_siteID];
    _searchText = @"";
    _searchResults = [[NSMutableArray alloc] init];

    [self setupHeaderView];
    [self setupTableView];
    [self setupConstraints];
    [self startObservingNotifications];
}
```