Fix for issue #1646, can’t dismiss comment reply keyboard

This fixes not being able to dismiss the keyboard when replying to a comment from the CommentViewController. Adds a tap gesture to dismiss, and the UIScrollView pan down to dismiss.

Also fixes scrolling issues in CommentViewController when the keyboard is shown over long comments.

I think it also fixes the same problem when replying in Notifications (#1646), but it's hard for me to test without being able to link up Jetpack.