Remove the fixed width from the annotation post button
and let the dropdown menu's width adjust to fit its
content rather than being the same width as the button.

To avoid the menu disappearing into the left edge of
the sidebar, the menu has been re-aligned so that
it is centered underneath the button's dropdown arrow.

 * Simplify the styling used to hide the dropdown menu
   to use just the visibility property instead of
   setting pointer events to none, applying a rotation transform
   _and_ setting opacity to 0.

   I'm unclear why the combination of a transform, pointer events
   and opacity was used originally but the dropdown menus
   continue to work with the simplified styling.

T-112