Preview

Changes

1. Implement Roadmap page suggestion #293, cc @rossberg
   a. Show proposal phase, standardization date and spec version in tooltips.
   b. Sort standarized proposals by their standardization date, and in-progress proposals by their start date.
   c. Slightly refactor the tooltip logic to make the above easier.
2. Add an error banner. Previously, unhandled errors could result in missing or half-rendered tables, creating confusion.
3. Break the footnote list into two columns. As it stands they feel too long and unbalanced.
4. Show the columns in a more sensible order: browsers, JS runtimes and WASM runtimes.
5. Clean up images.

Closes #293.