Automatically add favicon host to CSP if present

[HeySora]

Fixes #1890

• Tested with an empty favicon URL (-> No difference)
• Tested with a relative favicon URL (e.g. /favicon.ico) (-> No difference)
• Tested with an absolute favicon URL (e.g. https://my-other-domain.com/favicon.ico) (-> Adds my-other-domain.com to img-src)
• Tested with an absolute favicon URL containing a user and password (e.g. https://user:[email protected]/favicon.ico) (-> Correctly adds my-other-domain.com to img-src)
• Tested with gibberish (e.g. dfkgjhdf) (-> No difference)

[master3395]

Would this fix the current issue where there's no Favicon at all, even if it has been set?

I have set it like this:
https://newstargeted.com/hotlink-ok/favicon.ico

The user page is on the left and the admin page is on the right, with no icon.

[HeySora]

@master3395 yes, exactly!

the manual fix would be to go in the "Config" tab, look up content_security_policy, and setting its value to img-src newstargeted.com. that PR would solve that issue automatically without this additional step.