Bug 1026932: Edit button does not gray out when the folder is empty

This does not seem needed. I believe the idea is that we just need the .disabled on the buttons themselves instead of this class addition. 

Also, we can remove the .bottom-btn.disabled {} style from common.css, as this was the only code that it would have been used for, as there are no more "disabled" class use in the email app after this change.