Setting the `aria-pressed` attribute on a button indicates to assistive
technology that it is a toggle button. Because the `Button` component
always set this attribute and the `isActive` prop had a default value of
false, this meant that every `Button` in the UI was identified as a
toggle button by screen readers.

Fix the issue by renaming `isActive` to `isPressed` and aligning it with
the behavior of `aria-pressed`:

 - If `true`, the button is a pressed toggle button
 - If `false`, the button is a non-pressed toggle button
 - If undefined, the button is a regular button and `aria-pressed` is
   not set