FIX: navigate to topic/PM regardless of Shift key press (PR #15551)

Currently when pressing Shift key and hitting Reply button the user stays on the post they are on and does not get navigated to newly created topic/PM/reply. This is fine for replies but creates confusion when composing a new topic/PM.

This commits makes it such that pressing Shift key and Reply button (or ctrl-shift-enter / cmd-shift-enter) works only for replies and not for new topic/PM. The user will always be navigated to new topic/PM.


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