Add functionality to expand the markdown editor

The editor will now have a default height of 300px but a button will let
the user expand the editor. It will expand it to fit all content (or up
to 400px if the content was not exceeding 300px). In the expanded mode
the editor will keep growing as the user types more content.

config/locales/de.yml

@@ -835,6 +835,9 @@ de:
           <br>
           This mail was automatically sent by CodeOcean. <br>
         subject: "Eine Aufgabe auf CodeOcean benötigt Ihr Feedback"
+  markdown_editor:
+    expand: Editor aufklappen
+    collapse: Editor zuklappen
   request_for_comments:
     click_here: Zum Kommentieren auf die Seitenleiste klicken!
     comments: Kommentare

config/locales/en.yml

@@ -835,6 +835,9 @@ en:
           <br>
           This mail was automatically sent by CodeOcean. <br>
         subject: "An exercise on CodeOcean needs your feedback"
+  markdown_editor:
+    expand: Expand editor
+    collapse: Collapse editor
   request_for_comments:
     click_here: Click on this sidebar to comment!
     comments: Comments