Bug 905273 - [Messages] Recipients list pull down animation is inefficient r=julienw

In order to use the more efficient `transform` attribute for animating
the pull-down, the element responsibilities must be altered.

The top-level "#messages-to-field" serves as a basis for positioning the
"#to-label" and "#messages-contact-pick-button". It is no longer
animated, and because of this, it no longer draws the component's
background or border.

The `transform` attribute of the "#messages-recipients-list" is now
animated. Because of this, it draws the component's background and
border.

The JavaScript `generateHeightRule` function no longer has to reset the
height of the widget: all elements are now rendered at full height. The
method has been simplified to simply set the "#message-recipients-list"
element's `max-height` in order to trigger the rendering of a scroll bar
when the number of recipients causes overflow in multi-line mode.