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.

client/components/help/ShortcutsHelp.md

@@ -15,6 +15,7 @@ All the available shortcuts are listed below, grouped by page.
 | `→`                  | Next submission        |
 | `shift` + `←`        | Previous ungraded      |
 | `shift` + `→`        | Next ungraded          |
+| `a`                  | Approve feedback       |
 
 ### Students