Resolve "Importing grades" (1)

Closes #6 (closed)

What does this MR do (summary)?

Adds the option to import grades for an assignment.

Screenshots

What changes have you made (detailed)?

Does this MR meet the acceptance criteria?

• The main application was run to make sure the server still runs without errors.
• Tests were written to verify the behaviour of this code.
• Javadoc and other comments were added to make the code understandable.
• The code adheres to the style guidelines.