Fix rendering <a> without href when scheme unsupported

[Gargron]

Close #13037

This replaces an <a> that has no href with its children. But maybe we don't want any children elements like <span> to stick around?

[ClearlyClaire]

If the link has Mastodon-style formatting, it will hide the invisible parts and apply ellipsis, making the link unusable. So there needs to be some extra processing, or the CSS needs to be reworked.

[ClearlyClaire]

I'm not opposed to removing the <a> tag entirely, but it seems more complicated and error-prone than the purely-CSS solution.

Also, a <a> tag with no href attribute is semantically fine according to the HTML 5.1 specs (it says it must not have a target attribute though).

[ClearlyClaire]

seems good overall, except for the inlined nitpicks

also, I'm not sure it makes sense to allow all the supported protocols (which shouldn't be called HTTP_PROTOCOLS anymore) in embed and iframe tags

[ClearlyClaire]

This whole thing is done without fully parsing URLs in Sanitize
cf. https://github.com/rgrove/sanitize/blob/master/lib/sanitize/transformers/clean_element.rb#L143

It's probably safer (in terms of possible changed behaviors) and more CPU-efficient to do it the same way.

Btw, I think we should either reject URLs or rewrite them to be relative to the original status (an option I prefer, but is maybe not worth it).