[pgAdmin4][Patch]: RM #3029 Cannot sort columns when importing data

Hi Hackers,

Attached is the patch to fix the RM #3029 Cannot sort columns when
importing data.
Select2 orders the selections based on the order of the `<options>` behind
the scenes. This means that the order will typically match the order of the
options as they appear in the dropdown. For *import/export* it shows in the
order of columns define in the table.

Solution: I have just reorders the options inside the select2 every time,
an item is selected. I have made this change into the select2 control with
new option "*preserveSelectionOrder*" by default it is false, so won't
impact any other module.

--
*Akshay Joshi*

*Sr. Software Architect *

*Phone: +91 20-3058-9517Mobile: +91 976-788-8246*