Fix padding for GlDropdown

What does this MR do?

This MR fixes the padding for GlDropdown options to conform with design specs.

• The changes make sure there's always 16px vertical and horizontal padding between the dropdown elements

This was discussed at gitlab!51957 (comment 490020187) /cc @annabeldunstone

Screenshot

Before	After

Conformity

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• Added the ~"component:*" label(s) if applicable.