Implement http-equiv refresh

[pshaughn]

WPT html/webappapis/dynamic-markup-insertion/opening-the-input-stream/abort-refresh-immediate (some cases) and abort-refresh-multisecond-meta expect <meta http-equiv="refresh" to do its thing as in https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-http-equiv-refresh . Currently, http-equiv is commented out at the IDL level.

#25068 is the same missing behavior, but as triggered by the never-formally-standardized Refresh response header instead of the http-equiv. (As I understand it, once upon a time http-equiv literally just meant "behave as though you saw this response header" at first, then diverged as header semantics changed out from under it; its legal values still mostly do the same thing as headers of the same name.)

[pshaughn]

https://html.spec.whatwg.org/multipage/semantics.html#attr-meta-http-equiv-refresh gives us permission to "do nothing" in step 13, so failing to auto-refresh doesn't mean we're out of spec compliance.

On the other hand, this feature is actually used on the web: it's the easiest way to make a functional "redirect" in a shared hosting context when you can only upload static html files and not scripts.