[Dropdown] Add border-radius to dropdown items

Created by: royklutman

When setting the dropdown menu's padding to 0 ($dropdown-padding-y), the first and last dropdown items overlap the dropdown menu:

This is simply fixed by setting the border radius on the first- and last child of dropdown-item:

.dropdown-item {
  &:first-child {
    @include border-top-radius($dropdown-border-radius);
  }

  &:last-child {
    @include border-bottom-radius($dropdown-border-radius);
  }
}