Remove arbitrary HTML from header and footer

[Lewiscowles1986]

Arbitrary HTML could be removed frontend before sending to sandbox by creating a fragment from the css and js segments and disallowing any visible content.

This would make the plugin compatible with a wider range of themes and plugins, assisting in reducing the prevalence of themes and plugins in #7

1. Upon successful response body
2. Create a fragment with a wrapper element we can address from (reduces false positives from nested elements)
3. create iterable discover list querySelectorAll within the fragment using wrapper as namespace with only direct decedents on any non script, link, style, template (block visible elements)
4. remove all these direct descendents
5. get innerHTML of the fragment wrapper
6. send this to sandbox

Technically this means after receiving a response, parsing and modifying that response.

The reason I think this is best done Frontend is because it has a Real DOM to work with avoiding PHP, libxml and other parsing framework issues.

Drawbacks are additional complexity and iteration.