Bind to specific elements instead of random

this.$container can be whatever. It'll be whatever node
.trigger( 'flow_init' ) is called upon.
On init, that is $( document )
After moderation, that'll be .flow-post or flow-topic-container

Because this.$container is not consistent, we can't rely on it
to .on (and especially) .off event handlers. Initially, we bind
to $( document ), then after moderation, we try to .off existing
handlers on e.g. .flow-post, and bind the event anew.
Result, no handlers were removed (there was nothing to .off on
that element), and the handler is bound a second time, to
another node.

End result: things like double-collapse (immediately expand/
collapse) because a click is handled twice.

Change-Id: I5c45b53bceb54386901cf24526d46383ada25804