Allow running farming-hell in multiple tabs

[SquidDev]

Currently you can only really run the app in one tab at once - if you open it in multiple ones, changes you make in one will override ones made in another. Ideally we'd sync data between all tabs, so changes made in one show up in others.

Basically:

• Listen to StorageEvents.
• When we receive an event, reload the store - we'll have to do some level of diffing here (probably just on the servant/item level) as otherwise this'll re-render the whole page.
• Somehow get Formik to reload forms which haven't been touched at all. Unfortunately it appears changes to the initialValues prop aren't applied unless we specify enableReinitialize, but it's not clear if that's the behaviour we want either - to investigate!