Vertically Align Add and Cross signs using flex instead of margin

[ArashRasteh]

I am using SlimSelect for a project and I had made some changes to the style of the fields to match the other fields on the project. I realized that the add (plus) and cross sign (when opened) were not centered, so I made 3 changes to use flex to not only center the button when the field is a single line, but also when too many selections are made and it becomes multilined.

All the changes are to the ss-add class. The changes are:

• removed top margin
• set flex-direction to column
• set justify-content to center