Manage iframes

[over-engineer]

Expanding on #37, we should implement a mechanism to control iframes.

A few use cases where third-party iframes may set cookies include embedded:

• YouTube videos
• Vimeo videos
• Twitch streams
• Google Maps
• …

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A few key points to keep in mind:

• All iframes should be completely removed initially
• Users of the plugin should be able to set a (translatable) notice relative to that service (e.g. a different notice should be displayed for YouTube and Vimeo)
• Iframes should be loaded only after the site visitor has given consent
• Since we’re already using the Cookie Consent JS library by Orest Bida, we should consider using their iframemanager JS library (docs on integrating IframeManager with CookieConsent)

(cc: @formaciongrafica)