Keep rating history

[lrem]

It won't hurt to store players' skill ratings over the history. As a bonus, it will allow faster recalculation after removing a past match.

[lrem]

The display actually works, two more things I would like to do before calling it done:

• Horizontal lines at μ±σ.
• Discard the first n (3?) matches of unstable ranking, controlled by some checkbox.

[lrem]

Additional work items:

• Prevent it from abbreviating "1145" as "1.1k".
• Add an extra data point of (now, current_rating).

[lrem]

Discarding the first matches is not a good idea - we do want to see all the aberrations.

[lrem]

Removing the annoying abbreviation is blocked on https://github.com/google/google-visualization-issues/issues/2143.