HTML Sanitizer: More default settings for different use-cases

[HeyJoel]

In the default template project, the HTML editor allows you to add an embedded video, however the HTML sanitizer strips the iframe from the output.

HTML Sanitization always needs to be considered and should be a part of the template, but perhaps we can make it a bit easier here by providing several more discoverable defaults e.g. "Strict", "Lax", "Balanced" to allow users to opt-in to less secure sanitization where they trust the source or accept the risk.

Also mentioned is using bb code, or some other run-time parsable code for things like embeds.

Originally posted by @HeyJoel in https://github.com/cofoundry-cms/cofoundry/issues/422#issuecomment-814287426