Make basic Juypter notebook rendering improvements and roughly support folding unchanged context

Summary:
Depends on D20843. Ref T13425. Add very basic support for "Show Hidden Context", in the form of folding it behind an unclickable shield. This isn't ideal, but should be better than nothing.

Prepare for "intraline" diffs on content blocks.

Fix newline handling in Markdown sections in Jupyter notebooks.

Remove the word "visibile" from the codebase.

Test Plan: {F6898192}

Maniphest Tasks: T13425

Differential Revision: https://secure.phabricator.com/D20844

src/applications/files/document/PhabricatorJupyterDocumentEngine.php

@@ -307,7 +307,8 @@ final class PhabricatorJupyterDocumentEngine
         return $this->newCodeOutputCell($cell);
     }
 
-    return $this->newRawCell(id(new PhutilJSON())->encodeFormatted($cell));
+    return $this->newRawCell(id(new PhutilJSON())
+      ->encodeFormatted($cell));
   }
 
   private function newRawCell($content) {
@@ -328,8 +329,9 @@ final class PhabricatorJupyterDocumentEngine
       $content = array();
     }
 
-    $content = implode('', $content);
-    $content = phutil_escape_html_newlines($content);
+    // TODO: This should ideally highlight as Markdown, but the "md"
+    // highlighter in Pygments is painfully slow and not terribly useful.
+    $content = $this->highlightLines($content, 'txt');
 
     return array(
       null,
@@ -514,15 +516,20 @@ final class PhabricatorJupyterDocumentEngine
     return $label;
   }
 
-  private function highlightLines(array $lines) {
-    $head = head($lines);
-    $matches = null;
-    if (preg_match('/^%%(.*)$/', $head, $matches)) {
-      $restore = array_shift($lines);
-      $lang = $matches[1];
+  private function highlightLines(array $lines, $force_language = null) {
+    if ($force_language === null) {
+      $head = head($lines);
+      $matches = null;
+      if (preg_match('/^%%(.*)$/', $head, $matches)) {
+        $restore = array_shift($lines);
+        $lang = $matches[1];
+      } else {
+        $restore = null;
+        $lang = 'py';
+      }
     } else {
       $restore = null;
-      $lang = 'py';
+      $lang = $force_language;
     }
 
     $content = PhabricatorSyntaxHighlighter::highlightWithLanguage(