Requested viz updates

[joverlee521]

Internal review from Slack:

• [x] start plots 10 days after first estimate from model to avoid boundary issues where Rt values are artifactual (viz-updates branch)
• [x] cases estimates are off for Netherlands (resolved in #11)
• [x] smoothing of Rt needs to be updated to match notebooks (resolved in #13)
• [x] omit pivot category in Growth Advantage plots and update panel abstract to insert pivot category (viz-updates branch)
• [x] display frequencies y-axis as percentages, e.g. "10%", "20%" instead of "0.1", "0.2" (viz-updates branch)
• [x] use sans serif Helvetica font for static plots (viz-updates branch)
• [x] change ordering of plots (should match the order of static plots in nextstrain.org) (viz-updates branch)
• [x] change Rt point estimates to one or two significant digits (viz-updates branch)