Toggling pregrading and Identifying blank solutions

This merge request does two things:

• Automatically identify blank solutions (either multiple choice or not) and set a feedback option named 'blank' for each one.
• When creating an exam, there are three options to specify which questions should be graded automatically.
• Pre-grade a question again if a submission is reuploaded and is automatically graded (by 'Zesje')

The grading policy is specified in the following way:

• 0/set_nothing: Manually approve everything
• 1/set_blank: Automatically approve blank solutions
• 2/set_blank_single: Automatically approve blank solutions or solutions with one answer

Add a new field to a problem in the database with this number.

For a more detailed description, see #61.

Closes #33 and #61.

zesje/blanks.py

@@ -15,10 +15,10 @@ def set_blank(copy_number, exam_id, dpi):
     pages = extract_images(pdf_path, dpi)
 
     for image, page in pages:
-        save_image(np.array(image), page, dpi, exam_id, output_directory)
+        save_image(np.array(image), page, dpi, output_directory)
 
 
-def save_image(image, page, dpi, exam_id, output_directory):
+def save_image(image, page, dpi, output_directory):
     """Save an image at an appropriate location.
 
     Parameters