Add padding to select2 container displayed above

Added padding to ".select2-search" container instead of margin for ".select2-search input". This fixes bug of "#select2" movement when resizing page.

Steps to reproduce the bug:
- open select2 so it appears above the input
- resize the window

Actual results:
- #select2-drop moves up by 4px

Expected results:
- #select2-drop stays in same place