R1:01f277cef22c

Fix a CalendarExport issue when an existing export has an unsupported mode

Summary:
See D16676. When an export has an unsupported mode (bad database value, out-of-date object, etc) the intent of this code is to put it into the `<select />` so that you can save the form without silently changing the object.

However, it incorrectly calls `array_shift()` instead of `array_unshift()`.

Test Plan:
Edited a Calendar export with an invalid mode, saw the mode appear properly in the dropdown:

{F2957321}…