Don't show cancel confirm dialog when replying

When replying to a post (rather than topic title) the textarea
would be prefilled with [[User:Foo|Foo]]:, but when canceling the
form the reply callback would set it to a blank string.  Any value
other than node.defaultValue causes the cancel confirm dialog to
be displayed.

Change-Id: I50180f146a74e838c99824c23e417bf6419261b3
Merge "Don't show cancel confirm dialog when replying" into frontend-rewrite