Handle legacy code blocks without `<code>…</code>`

[rajveermalviya]

The current codehilite HTML wraps text or syntax-highlighted span elements inside <code>…</code> tags. However, older messages (the latest example found from April 2020 on CZO, so any message before then) don’t include this <code>…</code> wrapper, which is currently expected:

zulip-flutter/lib/model/content.dart

Lines 996 to 998 in 33c365a

	final grandchild = child.nodes.last;
	if (grandchild is! dom.Element) return null;
	if (grandchild.localName != 'code') return null;

Related discussion:
https://chat.zulip.org/#narrow/channel/243-mobile-team/topic/older.20codehilite.20blocks.3B.20html.20without.20.3Ccode.3E.3C.2Fcode.3E/near/1968837