fix: only linkify http and https URIs on the error page

escapeButPreserveUris() matched the scheme with "[A-z]+?://", which accepts
any scheme. A frame comment containing javascript://x.co/?%0Aalert(1) survives
htmlspecialchars() untouched and was then wrapped in an anchor, so clicking it
executed script in the origin of the error page. "[A-z]" is also a code point
range rather than a letter class, so it additionally matched the six characters
between "Z" and "a", making "_://x.co/a" a link too.

Match the scheme with "https?" instead, case-insensitively so uppercase
"HTTP://" keeps working. Other schemes stay as escaped plain text. The editor
integration links are not affected, they are emitted directly by the template
and never pass through this method.

Fixes #791

tests/Whoops/Util/TemplateHelperTest.php

@@ -63,6 +63,49 @@ class TemplateHelperTest extends TestCase
         );
     }
 
+    /**
+     * @covers Whoops\Util\TemplateHelper::escapeButPreserveUris
+     */
+    public function testEscapeButPreserveUrisIgnoresNonHttpSchemes()
+    {
+        // None of these contain HTML special characters, so escaping leaves them
+        // untouched and the only question is whether they get turned into links.
+        $originals = [
+            // Clicking such a link would run alert(1): the browser decodes %0A to a
+            // newline, which ends the "//x.co/?" JavaScript comment.
+            'javascript://x.co/?%0Aalert(1)',
+            'JAVASCRIPT://x.co/?%0Aalert(1)',
+            'vbscript://x.co/?%0Amsgbox(1)',
+            'data://text.html/?%0Ax',
+            // "_" is between "Z" and "a" in ASCII, so it used to pass as a scheme.
+            '_://x.co/a',
+            'See javascript://x.co/?%0Aalert(document.cookie) for details',
+        ];
+
+        foreach ($originals as $original) {
+            $this->assertSame($original, $this->helper->escapeButPreserveUris($original));
+        }
+    }
+
+    /**
+     * @covers Whoops\Util\TemplateHelper::escapeButPreserveUris
+     */
+    public function testEscapeButPreserveUrisStillLinksHttpUris()
+    {
+        $originals = [
+            'http://example.com/a',
+            'https://example.com/a',
+            'HTTPS://example.com/a',
+        ];
+
+        foreach ($originals as $original) {
+            $this->assertSame(
+                "<a href=\"$original\" target=\"_blank\" rel=\"noreferrer noopener\">$original</a>",
+                $this->helper->escapeButPreserveUris($original)
+            );
+        }
+    }
+
     /**
      * @covers Whoops\Util\TemplateHelper::breakOnDelimiter
      */