Add a way to modify CSP rules within an extension

[aledeg]

This will allow to change CSP rules to authorize the use of external scripts. We might need to add some safeguard since it will be virtually possible to load any script even malicious one.

Changes proposed in this pull request:

• Add support to extension CSP rules

How to test the feature manually:

1. Modify the CSP rules in an extension as described in the documentation
2. Validate that the new rule is appended to the existing rules

Pull request checklist:

• clear commit messages
• code manually tested
• unit tests written (optional if too hard)
• documentation updated

Additional information can be found in the documentation.

[math-GH]

Would it make sense to make it more transparent if an extension is adding external sources to the CSP?

[Alkarex]

Would it make sense to make it more transparent if an extension is adding external sources to the CSP?

Extensions can do worse things than editing the CSP, so while that could be nice if done in a light way, I do not think it is urgent