Harden default Content Security Policy (CSP)

[Erasure5959]

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What does this PR aim to accomplish?:

This PR aims to harden the Content Security Policy (CSP) which ships with Pi-hole by setting the default source to none and explicitly allowing certain directives to be loaded. This is essentially a block-by-default approach.

Relevant GitHub discussion: #2734 (comment)

How does this PR accomplish the above?:

This PR hardens the Content Security Policy (CSP) by setting the default source to none. The current CSP scans as below:

The suggested CSP shows as below from the same scanner:

Link documentation PRs if any are needed to support this PR:

pi-hole/docs#1332

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[DL6ER]

I have not been able to test my proposed changes myself but I think that's how it should be

[DL6ER]

Thank you for your submission!