Add drag and drop and improve "Choose a data source" design and behavior

[lynwilhelm]

Background:
This work was initiated by Lyndsay to improve the design of the CSV (table) example, content, and add easier way to select a data source (drag and drop)

Acceptance criteria:

• New drag+drop functionality (can accept single or multiple files at once)
• Load button location and behavior (only show when there is something to load)
• Apply (max width) to file selection area section
• New example CSV styling
• Reordering and revisions of content per design
• Add another data source behavior (only show when there is something to load)

LINK TO DESIGN