Adds MediaExcerpts and Appearances

This release contains:

• Adds MediaExcerpts (https://docs.howdju.com/concepts/media-excerpts)
• Adds Appearances (https://docs.howdju.com/concepts/appearances)
• Adds basic FactCheck functionality (https://docs.howdju.com/concepts/fact-checks)
• Adds basic quotation autoconfirmation
• Adds context to entities such as:
  • Number of appearances for a Proposition or MediaExcerpt
  • Justifictions targeting and using a Proposition.
• New documentation site (https://docs.howdju.com).
• Adds searching by domain and URL
• Migrates WritQuotes to MediaExcerpts in the data layer.
• Upgrades react-md to v2

This release also marks the complete migration of the web app to TypeScript! This milestone unblocks further improvements by greatly reducing the risk of fundamental code changes.