Replace all instances of Dropdown with ListBox, and remove the Dropdown component

[mkrause]

• Resolves Improve Dropdown and Select components #127.
• Resolves Banner: text overflow word breaking not working correctly #161

TO DO:

• When the dropdown menu is open, and the user hits "Enter", the focused item should be selected (and the menu closed).
• Fix visual issues in ListBoxLazy (e.g. empty placeholder)
• Fix the issue where the ListBoxLazy infinite scroll will load the next page even when the user has not yet scrolled down.
• Update SolutionSelector: solutions should be a named prop, children should be an optional render prop for the selected content.