Roadmap improvements

[andylizi]

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.

[rossberg]

This is great, thank you!

[tomayac]

I think it's okay-ish to close this, since the table was since restructured to add phase information.

I agree not everything that was suggested in #333 and #293 is reflected, but the team is super constrained and the current iteration of the table seems to get the job done. If you disagree heavily, please reopen.