[Dropdown] Add border-radius to dropdown items

[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);
  }
}

[MartijnCuppens]

If you zoom in, you can see the border radii are not perfect:

We'll need to apply the same technique as with the cards here:

bootstrap/scss/_variables.scss

Line 796 in 73a98d9

$card-inner-border-radius: calc(#{$card-border-radius} - #{$card-border-width}) !default;

@royklutman, can you change that?

[royklutman]

@MartijnCuppens Great catch, thanks for noticing. I'll update the PR.

[MartijnCuppens]

Perfect!