simplify and improve encapsulation of CompletionWidget CSS

Created by: sqs

• Eliminates need to pass constant styles in web app in favor of wrapped component
• Removes <div style={...}> in favor of CSS
• Removes scrollbar on web app that extended above the menu
• Adds basic React component snapshot tests
• Addresses review comments:
  • https://github.com/sourcegraph/sourcegraph/pull/3392#discussion_r275943011
  • https://github.com/sourcegraph/sourcegraph/pull/3392#discussion_r275894276
  • https://github.com/sourcegraph/sourcegraph/pull/3392#discussion_r275893667