Bug 1023658 - [User story] Hide CC and BCC fields while composing an ema...

...il
Generally I like to use event delegation, but in this case, since each click has very specific behavior, and we only need to bind two clicks, so not much more work, I like the idea of just binding a specific click handler to specific nodes, and avoid the extra selector walking/finding.

So perhaps add a class cmp-envelope-line-to and cmp-envelope-line-cc on the appropriate cmp-envelope-line divs, then in the constructor grab those and create specific `this` properties in the card instance to hold on to them, and have a dedicated click handler on each that does the on/off behavior.

Note that how this node binding is done will noticeably change once bug 1005446 lands in master (which is hopefully some time this week/early next week). I apologize, that may cause some issues for this pull request, needing to update to the new model. But in that model, it is easier to set up the bindings.