Set CSP in meta tag

[amaury1093]

Right now, in production mode, we are setting CSP on onHeadersReceived

https://github.com/paritytech/fether/blob/d0ae207176fd28b8e986e01499f749c4e676c40a/packages/fether-electron/src/main/app/methods/setupRequestListeners.js#L40-L53

However, this doesn't actually set the CSPs: https://electronjs.org/docs/tutorial/security#csp-meta-tag.

We need to manually add a <meta> tag in index.html