Fix Plainify edge cases

This commit replaces the main part of `helpers.StripHTML` with Go's implementation in its html/template package.

It's a little slower, but correctness is more important:

```bash
BenchmarkStripHTMLOld-10    	  680316	      1764 ns/op	     728 B/op	       4 allocs/op
BenchmarkStripHTMLNew-10    	  384520	      3099 ns/op	    2089 B/op	      10 allocs/op
```

Fixes #9199
Fixes #9909
Closes #9410

tpl/transform/transform_test.go

@@ -237,6 +237,7 @@ func TestPlainify(t *testing.T) {
 		expect any
 	}{
 		{"<em>Note:</em> blah <b>blah</b>", "Note: blah blah"},
+		{"<div data-action='click->my-controller#doThing'>qwe</div>", "qwe"},
 		// errors
 		{tstNoStringer{}, false},
 	} {