Restyle collections section for clarity, avoid tables.

[kimisgold]

• The current use of tables to display the collections and communities does not make a lot of semantic sense. I revised the markup to use divs and flexbox for layout.
• I also added more background color accents to help with readability, as requested in #72.
• Descriptions can get unwieldy, so I included collapsible toggles for them. For users who prefer to see them all, there is an "expand all" button, as well as an undo with "collapse all".