compose: Allow flatpickr to automatically choose the direction.

Previously the flatpickr was always set to show at the top but this
led to it being cut off when the message was at the top of the
screen -- should happen only when someone is editing a message.

web/src/compose.js

@@ -734,8 +734,8 @@ export function initialize() {
                 on_timestamp_selection,
                 get_timestamp_for_flatpickr(),
                 {
-                    // place the time picker above the icon and center it horizontally
-                    position: "above center",
+                    // place the time picker wherever there is space and center it horizontally
+                    position: "auto center",
                 },
             );
         }