Async data loading

[annehaley]

Resolves #2

This PR makes the following changes according to the suggestions in the above issue:

• Write data files with gzip and decompress them client side. Smaller files mean faster transfers.
• Fetch data files asynchronously with callbacks to fill in relevant parts of the page once each data file is received (rather than awaiting all three data files before filling in the whole page)

[wdconinc]

It might be useful to setup a netlify preview for pull requests. That way the testing can happen on remote servers instead of local servers (which will have faster serve times).

I will take a look later when I have a chance to pull a version of this branch for local deployment.