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)