File input improvements

[jrubenoff]

Outcomes

• If the user accidentally starts uploading a large file, there is an obvious way for them to cancel it.
• If the user's connection is lost while uploading a large file, they should receive accurate instructions to fix the problem.
• Provide greater visual polish for each visual state of the "upload file" interaction.

  Structure

• Make a cohesive UI component for file uploads, maintaining visual consistency between the default, progress, and finished states
• Add a cancel button to downloads in progress
• Build better error handling when the user's connection is lost

  Interaction

Still needs prototyping: