Update sky-dropdown to have appropriate z-index

[Blackbaud-PatrickOFriel]

the menu for sky-dropdown has no explicit z-index so it can be cut off by relative elements. We should add an appropriate z-index (which is able to handle things like the dropdown menu residing in a modal)

[Blackbaud-PatrickOFriel]

We can also look into making the dropdown menu position fixed with appropriate top, left, etc so that it handles not getting cut off by being inside of an overflow: scroll or overflow: auto element.

[Blackbaud-PatrickOFriel]

In 2.0.0-beta.9