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

@@ -26,7 +26,7 @@ def get_blank(problem, dpi, widget_area_in, sub):
 
 def set_blank(copy_number, dpi, output_directory):
     pdf_path = os.path.join(output_directory, 'generated_pdfs', f'{copy_number:05d}.pdf')
-    pages = extract_images(pdf_path)
+    pages = extract_images(pdf_path, dpi)
 
     for image, page in pages:
         save_image(np.array(image), page, dpi, output_directory)