[Feature]: Create catastrophic error page

[itsalaidbacklife]

Feature Summary

We should create and host a custom error page for when the entire site is down. This should be statically hosted somewhere outside of heroku so that it can be served when the application crashes

Detailed Description

Currently, when the cuttle server crashes, the hosting provider will show whatever application error page is the platform default. In the case of heroku (used for the official production deployment), that shows this page:

Instead, we should customize this behavior with a branded and cuttle-specific error page that shows the logo and explain that something went wrong and that users should come back later. This page should be statically hosted somewhere besides heroku (since this is served when the app is crashed). Ideally, the page should be included in the source code, for consolidated maintenance. Perhaps for this reason it should be hosted in github actions? We can investigate a build setup for statically hosting the page based on the latest code.

See the heroku docs for details regarding how to configure heroku's env vars to point to the html for an error page that's statically hosted elsewhere.