Add a CSP to the Dagit app.
- Lock down default-src and override with some 'self' and a few other minor needs.
- In prod usage, within app.py, generate a one-time nonce and insert into index.html. This is used as __webpack_nonce__ (https://webpack.js.org/guides/csp/), which can then be consumed by styled-components.
- In development, allow inline style and script since we won't have a nonce.