- Configurable via 'ALLOWED_IFRAME_SOURCES' .env option.
- Also updated how CSP rules are set, with a single header being used
instead of many.
- Also applied CSP rules to HTML export outputs.
- Updated tests to cover.
For #3314
Was causing things like emjoi within script content to be somewhat
mangled. Instead we force UTF8 only parsing via XML declaration.
Added test to cover.
For #2923