make disabled navigation buttons have aria-disabled attribute and remove tabindex if button is disabled to meet accessibility guidelines

[seemajune]

In order to meet WCAG 2.4.3 - Focus Order, inactive elements have to have the disabled attribute set to true in order to let users of assistive technology know that the element is disabled, adding aria-disabled when the button is disabled is also encouraged so that the element is not removed from the accessibility tree

https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Attributes/aria-disabled

https://stackoverflow.com/questions/62752054/swiper-js-w3c-aa-keyboard-tab-issue