Resolve "Endpoints for importing grades"

What does this MR do (summary)?

• Closes #121 (closed)
• Adds grade import endpoints
• Adds endpoints for getting all submissions by assignment and module
• Fixes a bug where Submission was not properly mapped to SubmissionDetailsDTO

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.