Revert to safe mode if the app explodes

[toolness]

Right now our browser-side error boundary just tells the user a fatal error occurred, but it'd be nice if we could revert to a "safe mode" where the server simply doesn't deliver JS assets to the client, and the client uses our app like it's 1995 (which they can do once #121 and #123 are fixed).

Prior work: https://github.com/18F/calc/pull/654

Note that at 18F the content team recommended we rename "safe mode" to "compatibility mode", which we did in https://github.com/18F/calc/pull/861.