Incorrect use of aria-pressed for sidebar collapse/expand button

[acelaya]

Part of #6151

The sidebar toggle button has an aria-expanded attribute and an aria-pressed attribute. The aria-pressed attribute does not change state and is not appropriate for this element.

Because this attribute is present some assistive technologies communicate the control as a toggle button, and the presence of this attribute potentially overrides the aria-expanded attribute in some cases (the element was communicated as expanded by NVDA but not by VoiceOver).

The control is also missing an aria-controls attribute that points to the ID of the content that it expands/collapses