Remove visitLinksForShortLinks features (#6257)

The visitLinksForShortLinks feature would look inside of an <a> tag and
run shortLinkProcessorFull on any text, which attempts to create links
out of potential 'short links' like [[test]] [[link|example]] etc...
This makes no sense because you can't have nested links within an <a>
tag. Specifically, the html5 standard says <a> tags can't include
interactive content if they contain the href attribute:

 http://w3c.github.io/html/single-page.html#the-a-element

And also defines an <a> element with a href attribute as interactive:

 http://w3c.github.io/html/single-page.html#interactive-content

Therefore you can't really put a link inside of another link. In
practice none of this works anyways since browsers won't render it, it
would probably be broken if they tried, and it is causing a bug
(#4946). No current tests rely on this behavior either.

This removes the feature and also explicitly excludes the
current visitNodeForShortLinks from looking in <a> tags.

modules/markup/html_test.go

@@ -222,4 +222,8 @@ func TestRender_ShortLinks(t *testing.T) {
 		"[[some/path/Link #.jpg]]",
 		`<p><a href="`+notencodedImgurl+`" rel="nofollow"><img src="`+notencodedImgurl+`"/></a></p>`,
 		`<p><a href="`+notencodedImgurlWiki+`" rel="nofollow"><img src="`+notencodedImgurlWiki+`"/></a></p>`)
+	test(
+		"<p><a href=\"https://example.org\">[[foobar]]</a></p>",
+		`<p><a href="https://example.org" rel="nofollow">[[foobar]]</a></p>`,
+		`<p><a href="https://example.org" rel="nofollow">[[foobar]]</a></p>`)
 }