Add render template hooks for links and images

This commit also

* revises the change detection for templates used by content files in server mode.
* Adds a Page.RenderString method

Fixes #6545
Fixes #4663
Closes #6043

hugolib/page_test.go

@@ -93,12 +93,6 @@ Summary Next Line. {{<figure src="/not/real" >}}.
 More text here.
 
 Some more text
-`
-
-	simplePageWithEmbeddedScript = `---
-title: Simple
----
-<script type='text/javascript'>alert('the script tags are still there, right?');</script>
 `
 
 	simplePageWithSummaryDelimiterSameLine = `---
@@ -325,6 +319,7 @@ func normalizeContent(c string) string {
 }
 
 func checkPageTOC(t *testing.T, page page.Page, toc string) {
+	t.Helper()
 	if page.TableOfContents() != template.HTML(toc) {
 		t.Fatalf("Page TableOfContents is:\n%q.\nExpected %q", page.TableOfContents(), toc)
 	}