Move files to src/container/

We have a bunch of top-level files in src/. A lot of them create objects that manage app state and are stored in `container`:

- alert.js
- unsaved-changes.js
- scroll-behavior.js
- i18n.js
- router.js
- routes.js

I think it'd be cleaner to move these files to their own directory. I'm thinking src/container/. Then src/container.js could become src/container/index.js.

I had this thought in mind while working on getodk/central-frontend#1067. I added src/container/hover-card.js to manage the state of the hover card. It's the first file in src/container/.