Improve element hiding empty check to avoid potentially hiding visible content

[dharb]

Asana link: https://app.asana.com/0/1203557590179903/1204090946699947/f
Corresponding privacy test page update: duckduckgo/privacy-test-pages#130

While we try to be precise in the selector rules we use for element hiding, we've run into a few cases now where we accidentally hid visible content that matched an ad selector. This PR adds several guardrails to prevent hiding visible content even when elements match common ad selectors.

Specifically, this PR:

• avoids hiding elements that contain various media tags such as <embed>, <object>, <audio>, and <map>
• avoids hiding elements that contain form tags such as <form>, <input>, <textarea>, <select>, <option>, and <button>
• avoids hiding elements that contain real images - we previously weren't counting images as visible content since ad containers frequently include tracking pixels and small ad logos (like adchoices). Now we the size of contained images and don't hide when they're > 20px width or height.

[jonathanKingston]

Can we make the selectors configurable?