Explicit prompt to "Load Sample Data"

[jmandel]

Right now we load sample data by default, every time the page is loaded. But given #4 from @sarahmcdougall we'll want to think about changing this behavior, so the home page does something more like:

1. On first load, show an empty vaccines list and a "Load sample data" button. Clicking this loads sample data from a static set into localStorage.
2. On subsequent loads, show the vaccines that have been saved localStorage (whether they came from sample data or user-scanned SHCs)
3. Below the vaccines list, show a "Clear all data" button that resets the state of the app