Add middleware and nonce support

[instification]

This is a major rewrite of the addon.

Main goals:

• Set a proper Content-Security-Policy header via middleware instead of via a meta tag
• Add nonce support
• Work in dev mode
• Fix issues with icons that set style= attributes

Main downsides:

• A number of volto files needed to be shadowed
  • helpers/Html/Html.jsx
  • components/theme/Icons
  • The server.jsx from volto will also need to be customised for the nonce to be properly set in the redux store. This could also be added as a customisation here.
• It was developed against an older version of volto so the methodology & customisations will likely not work with the latest version of volto

Todo:

• Tests
• Work with latest volto