Having the CSP built in a function instead of in the template makes it easier
to properly construct it. This was also the opportunity to switch from
default-src 'self' to default-src 'none', to deny everything that isn't
explicitly allowed, instead of allowing everything coming from 'self'.
Moreover, as Miniflux is shoving the content of feeds in the same origin as
itself, using self doesn't do much security-wise. It's much better to
systematically use a nonce-based policy, so that an attacker able to bypass the
sanitization will have to guess the nonce to gain arbitrary javascript
execution.
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