Add <iframe> element

[petele]

Used compute_from: html.elements.iframe since a number of the sandbox attributes are later in life

[foolip]

Caniuse has https://caniuse.com/iframe-srcdoc and https://caniuse.com/iframe-sandbox as separate features. These are quite old by now, but I wonder if we still shouldn't just create separate features for these for now? @ddbeck WDYT?

[foolip]

I've sent PRs for separate features for sandbox and srcdoc:
#1468
#1469

@petele can you exclude those keys from this feature?

[petele]

Removed sandbox and srcdoc, which means compute_from was no longer necessary, so removed that too.

[foolip]

@petele can you add back compute_from? Without it, the feature is much newer than it should be.