Make fields readonly once form is submitted

It's a bit upsetting that this relatively small change touches 24 files. Any suggestions for alternative implementations welcome.

There are still a few issues, not sure how high fidelity you want this:

• The styling of some elements still changes subtly on hover/focus.
• Clicking around the form occasionally triggers a PATCH request to the server as if the data had been changed.
• Server needs updating to reject any changes.

Closes #72 (closed)