Implement skeleton screens across the board

[peterzen]

Using https://github.com/buildo/react-placeholder, implement skeleton screens in the top level components.

• Implement placeholder components for subcomponents (as a rule of thumb, there should be one placeholder for <Card>...</Card> comps, in the same file.
• Wrap subcomponents with <ReactPlaceholder...>
• Some components data loading mechanics will have to be changed so that we have a loading prop that can be passed to ReactPlaceholder