Bug 847168 - [User Story] Email Signature

Hopefully final nit: even though I asked for as much to be moved into this mixin as possible, I now think this method should be called `updateSignatureButton` and then each card that mixes in this mixin, have its own `onCardVisible` that just calls `this.updateSignatureButton`.

This will help avoid accidental overrides if those cards need to do other work later during the `onCardVisible` hook that is called by the Cards infrastructure.