Fix build (dayjs import) for tests and production

[chaoran-chen]

If we run

npm run build
npm run test

we get the error

TypeError: dayjs.extend is not a function

Similarly, we get an error if we build the storybook with

npm run build
npm run build-storybook

Opening the prevalence over time plot, we get:

However, the local development instance of storybook works:

npm run build
npm run storybook