Get task description text for tasks with subtasks too

(cherry picked from commit 42ef9027)

python/educational-core/src/com/jetbrains/edu/learning/StudyUtils.java

@@ -421,7 +421,7 @@ public class StudyUtils {
   }
 
   @NotNull
-  private static String constructTaskTextFilename(@NotNull Task task, @NotNull String defaultName) {
+  public static String constructTaskTextFilename(@NotNull Task task, @NotNull String defaultName) {
     String fileNameWithoutExtension = FileUtil.getNameWithoutExtension(defaultName);
     if (task instanceof TaskWithSubtasks) {
       int activeStepIndex = ((TaskWithSubtasks)task).getActiveSubtaskIndex();

python/educational-core/src/com/jetbrains/edu/learning/courseFormat/tasks/Task.java

@@ -9,8 +9,9 @@ import com.intellij.openapi.project.Project;
 import com.intellij.openapi.vfs.VirtualFile;
 import com.intellij.util.xmlb.XmlSerializer;
 import com.intellij.util.xmlb.annotations.Transient;
-import com.jetbrains.edu.learning.checker.StudyTaskChecker;
 import com.jetbrains.edu.learning.EduPluginConfigurator;
+import com.jetbrains.edu.learning.StudyUtils;
+import com.jetbrains.edu.learning.checker.StudyTaskChecker;
 import com.jetbrains.edu.learning.core.EduNames;
 import com.jetbrains.edu.learning.courseFormat.*;
 import com.jetbrains.edu.learning.stepic.EduStepicConnector;
@@ -163,7 +164,12 @@ public abstract class Task implements StudyItem {
 
   public String getTaskDescription() {
     if (!taskTexts.isEmpty()) {
-      return taskTexts.get(EduNames.TASK_HTML);
+      String filenameHtml = StudyUtils.constructTaskTextFilename(this, EduNames.TASK_HTML);
+      if (taskTexts.containsKey(filenameHtml)) {
+        return taskTexts.get(filenameHtml);
+      }
+      String filenameMd = StudyUtils.constructTaskTextFilename(this, EduNames.TASK_MD);
+      return taskTexts.get(filenameMd);
     }
     return null;
   }