Add @mention suggestions to the "long form" iPad commenting flows

That is, the flows that include the "tall" (multi-line) EditReplyViewController, which is used on iPad for both notifications and me>sites>comments.  This fixes #2699 

Also avoided adding SuggestableView to IOS7CorrectedTextView per discussion with @jleandroperez 

Also tested to ensure this does not cause regression on ReplyTextView.
Common cocoa pattern is: `self = [super initWithNibName:... bundle:..]`
Reason to assign whatever the superclass returns, to self, is that the super-initializer might return a different object. 

(Plus: it's a good practice not to hit properties in initializers!).